That's what it was, don't forget how impossible it seemed at the time, it was a giant upset and shock.
> You think if Hillary went up against Trump, America would choose her 19 times out of 20?
If you are using "Hillary" and "Trump" figuratively about future elections with similarly matched candidates, then yes, see above.
I'm not saying she ran a bad campaign (though she did). I'm talking about her "negatives". Decades of scandals. (Yeah, Trump had them too, but at a minimum it meant that Hillary couldn't use Trump's scandals against him. Also, Hillary's scandals got a lot more national coverage when they happened than Trump's did.) Benghazi. The email server (and with it, the impression that she thought that rules were for other people). The impression that she thought that she was owed the presidency, rather than having to earn it. The way the DNC chose her over Sanders, overruling the will of many of the primary voters. And on and on.
It wasn't obvious at the time, because much of the press was pro-Hillary. But she was a terrible candidate. I think if the Democrats had run anyone else, they probably would have won against Trump.
Biden isn't disliked as much as Hillary was. In that sense, he's a better candidate. (Trump is disliked as much as Hillary was, and then some. Perhaps that was your point.) But Biden doesn't generate any enthusiasm, except that he's "not Trump". (In fairness, some of the support for Trump was that he wasn't Hillary.)
1. Trump is no longer an unknown.
2. Biden is far more popular than Clinton was at her peak.
3. Biden is male (which is sadly very relevant in US elections).
4. We are no longer in the biggest economic expansion in US history. We may be in a historic recession.
5. There is a pandemic that has killed 60x the number of Americans that died on 9/11, and it is no under control in the US at all.
6. Kamala Harris is not like Tim Kaine.
7. Lots of anti-Trump voters are afraid to be complacent this time.
8. Suburban women, true independent voters (those who dislike both candidates), and older Americans are either breaking for Biden or have substantially switched to Democratic support after going for Trump in 2016.
A better question would be: is anything the same this time around?
4. But the biggest (longest, anyway) economic expansion was largely under Obama. That wasn't a reason to vote for Trump in 2016.
The rest of your list I agree with, with the possible exception of 8. I'm not sure that we can tell. I think the climate is so polarized, and the Democrats have the microphone to such a degree, that people who support Trump aren't willing to say in public that they do so. But you could in fact be right. We'll see.
For #4 the economic growth was under Obama but it meant that at the time of the election people were (mostly) fat and happy; the cost of taking a chance seemed to be lower and it was easier to tell yourself that 'this guy is a businessman, he could not fuck up this strong economy and might even do better.' Now people can see what a catastrophic mistake it was to make this assumption, but I can almost understand the reasoning. I think the point of claim #4 was basically that the most that any candidate could claim about the economy is that they would have recovered it better or done it faster -- another important claim that could be made about it would be that you would make sure that the 'right people' got more benefits from the rising economy and not those elites; the basic populist economic playbook.
Only in your filter bubble.
I wouldn't take his not being prepared as evidence of anything.