Any climate policy other than a carbon tax (with tariffs on goods from non-participating countries) is going to be a failure - it will be shot through with loopholes and rebates, such that little reduction is actually achieved.
I don't think we do the import tariffs part though, unfortunately.
If you look at GHG emissions for the period and compare them to the rest of Canada, Ontario did better than average. Not perfect, of course, but to say that the only way to bring down GHG emissions is via a carbon tax is just not true.
From some quick googling that was in place from January 1st, 2017 to July 3rd 2018, while the Canada wide (sort of) carbon tax was implemented in the GHGPPA, which looks like it came into effect June 21, 2018. You have a sample period of ~1 year, it seems like you're comparing cap and trade to "nothing"? The difference provinces in Canada are vastly different in terms of geography, industry, power sources, other laws, etc.
My prior is that it's not going to be good evidence of basically anything, it was too short with no real controls, but I could be wrong.
And certainly I don't disagree that there are other ways to bring down carbon emissions... but I am skeptical that they will be particularly effective.
[1] Cap and trade dates: https://www.ontario.ca/page/cap-and-trade-ontario
[2] GHGPPA dates https://en.wikipedia.org/wiki/Greenhouse_Gas_Pollution_Prici...
For example in the EU, the carbon price had fallen too low, so credits were arbitrarily pulled out of the market to force the price up.