1. They're the ones that walked away.
2. They, as I pointed out in another thread, don't have the slightest idea how their own economy works.

The deal wasn't good enough for them. So they walked away.

That's actually the correct negotiating technique for someone like Trump. You always have to be willing to walk away from any negotiation, of course. But Trump almost always likes to retrade his "deals." As one of the officials put it, he signs his name in pencil. If you congenially agree to something with him, he reads that as indicating that he didn't squeeze you enough and you can get more.

And there's also the approach that you frequently mention - the best way to deal with a bully is to tune him up. Trump ripped up all the prior agreements with Canada because he could. The U.S. is a much stronger country than Canada, so they can't beat us in a "fight," whether a trade war or a real world. We're just more powerful than they are. But the way to deal with a rival like that (since we're not going to treat them like allies) is to make a stand, even if you'll get the worst of it. You have to make them pay a price in order to discourage them from continuing to take whatever they want from you.

Trump brought this pain on us, of course, by being "bombastic and unnecessarily antagonistic" as you put it. By doing that, he created so much political support for pushing back against the U.S. that Carney has almost unlimited freedom of movement, even though this will cause domestic pain for Canada. The Canadian public is so mad at the U.S. right now that Carney can be as aggressive as he wants, and won't pay a political cost - he'll probably even benefit. Meanwhile, Trump doesn't have that at home. There's no domestic appetite for squeezing Canada more than the last deal (which we agreed to!), and Trump didn't invest any political capital into preparing the electorate politically. They're trying desperately to backfill that with all kinds of after-the-fact rationalizations - anything other than Trump likes both tariffs and exercising domination over those who annoy him - in order to try to get some political support for this.

Not sure our efforts will work, but Carney's refusal to take Trump's offer is pretty smart. It will damage the Canadian economy, to be sure - but probably not as much in the long run as taking the deal and just "praying Trump doesn't alter it any further," to quote a certain Sith Lord. In the meantime, Trump and the GOP are going to deal with some political fallout they'd prefer not to have to have on their plate right now, which puts Carney in a better spot when talks resume.