So...the "strategic ambiguity" argument goes against allowing the Ukraine into NATO. After all, if a "will they or won't they approach" is good enough for China - who views Taiwan as a runaway province and doesn't even recognize Taiwan as a country - then why wouldn't than be good enough for the Ukraine?

It could be - I don't think anyone has said that NATO membership is the only way to guarantee Ukraine's security. There are many reasons why it's both a good and bad idea, but it's not the only solution.

But that's part of why Trump's come under so much criticism from Ukraine's defenders. The essential element of a Taiwan-like solution (apart from memorializing it in a treaty) is unwavering public commitment to the security policy. You have to make sure that the other side holds no misconceptions about the willingness and capability to defend the smaller country.

That doesn't jibe well with Trump's conception, in practice, of "America First" - if Trump is willing to let Russia get all of its war aims in Ukraine despite being the unprovoked aggressor, Russia's certainly not going to regard the U.S. as a likely defender of Ukraine if they decide to eat the rest of it down the road. And that ratchets up the danger for Taiwan as well; if the U.S. is willing to live with just token consequences for Russia attacking and seizing sovereign territory in their sphere of influence, China may conclude that the same is on offer in Taiwan.