This is patently false, bordering on scare-mongering.
Canada does 80% of their trade with the US, who has much more power than them.
The shelves are never empty in Canada.
Every other nation in fact is largely in the same boat: Australia, New Zealand, Japan, Singapore - they all depend on partners for trade.
It works, and the shelves are never empty.
In addition the US and Canada have a customs union with smooth flow of good without lengthy border checks - AND - at the same time are free to develop their own foreign trade policies and deals.
Canada could theoretically increase their GDP and literally everyone of them would be a several shades richer immediately by 'joining the USA' but it's not exactly a movement.
Respectively, there's no reason that the UK couldn't have something similar with the EU - and for some reason there's all sorts of hyperbole to suggest somehow it couldn't work.
UK citizens voted to join the EEC, and never really did for the EU, if there was a referendum on the Treaty of Lisbon in 2007 that passed in the UK ... I bet Brexit would have never happened - there'd have been too much legitimacy in a recent vote and the leaders could not have justified a referendum.
The 'empty shelves' / 'rationing medicine' memes are ridiculous.
The EU is 80% about trade, it always has been, and very few people had existential qualms with that. There would hardly be such a thing as a 'Eurosceptic' if they focused on that.
GDP might increase for Canada overall by joining the USA, but portraying it as having no economic downsides for Canadians is disingenuous.
Every trade deal / national-merger has upsides and downsides.
The challenge is to see them clearly.
There is quite a lot of scary rhetoric about 'trade failure' about Brexit that is frankly politicized, because outside of some temporary disruption, trade will go on. Caveats - yes. Failure - no.
Moroever, nobody seems to look at longstanding deals that work at least in some ways, particularly NAFTA/USMCA.
In particular, I find it really problematic that the notion of sovereignty is portrayed as 'xenophobic' in the case of Brexit, but 'idealized' in the case of Scottish indyref. And of course more theoretically, the notion of 'Canada joining the USA' (which is not as outrageous at it seems, it almost happened a few times only a few generations ago!) - would be find stupendous outrage among Canadians. Not known for their over nationalism, Canadians would give you a serious earful about that ... and nobody would bat an eye or think there's something wrong with it. The UK is not Scotland and is not Canada, of course, but the hyperbole about the issue is unwarranted.
Underneath the outrage again, lays bare the fact that the EU is mostly a 'really good economic deal' whereas the rest of it is fuzzy.
Perhaps the most 'personally impactful issue' - movement - could easily be achieved with a simple work program. NAFTA/USMCA allows anyone with specific accreditation to work across the border for 6 years with a stamp at the border - easy peasy. Something along those lines could have been achievable by the EEC without having to go full EU. I'm not saying it's ideal, or even one way is better than the other, rather, that the issue is overly politicized in the press, and the EU-specific union is too oft presented as 'the only solution' , when really, it's not. There are other ways.
In fact, given a post COVID world, and that we now have true real-time digitization ... it may be possible to even move away from the Euro and go to 'National Euros' - basically to give back the power of monetary policy to individual nations (which is a huge constraint right now, probably the #1 problem that even pro-EU nations have) while at the same time achieving a pan-European financial policy. This would be one example of a truly modern approach to currency, but it can't happen if 1/2 of the political forces are pushing for integrated political union.
>Canada could theoretically increase their GDP and literally everyone of them would be a several shades richer immediately by 'joining the USA'
This is 'basic communication'.
In much the same way many people are making the argument 'Brexit will cause UK citizens to be poorer' (however debatable in the first place) - obviously doesn't imply that the case will be the same for all UK citizens.
Finally - to your specific point - Canadian famers would be economically better off financially, because there would be a massive buying spree for all assets that were otherwise protected.
If they tried to 'hold out' of course they'd be put out of business, but from a raw economic value perspective, they'd be 'richer' no doubt.
CIBC/BMO/Desjardins - all the major banks, Bell/Telus/Rogers (Telcos), CTV/Global (networks), Loblaws (Groceries) etc. etc. would immediately be acquired or merged into larger American entities. (Except in the rare possibility that the 'smaller' Canadian entity made a deal with a Hedge Fund for a massive leveraged buyout of their much larger American peer, but that's unlikely)
Of course, one might argue that would be a 'giant disaster' for Canada, even as Canadians had immediately higher incomes and GDP/capita - and the argument is essentially derived from nationalism. Hence, the hypocrisy of people lamenting that the UK doesn't deserve a degree of material sovereignty.
I think there are some other areas that are needed they could focus on, a European defense pact for example with joint exercises outside of NATO now that the US is AWOL.
I think the euro was not a great idea, and I’m not sure in the long run if 100% freedom of movement was smart either.
I would absolutely call an attempt to equate those two things as "disgustingly misleading".