I strongly disagree. If China called and said, "Oh hey, we're gonna nuke Taiwan, but it's cool, right?" I think we would absolutely threaten nuclear retaliation. Or if Russia did the same with...well, about anybody.
Now, if it was a surprise strike, it would get a lot thornier because people would have time to do the cost/benefit analysis of "Do we want to make this worse?" vs. "Can we allow this to go unpunished?"
If you take our nuclear strikes and focus on a simple military strike it becomes a more contentious issue. If China launched an invasion of Taiwan would the USA respond with nuclear force? This issue faced France during the cold war, wherein they were concerned that in the event of the Soviets launching a rapid invasion through Germany into France, the USA would prefer to have a military defeat in Western Europe than to start a nuclear exchange.
Honestly? Depends on who's in charge. With Trump, all bets are off.
10-20 years ago, would have been a no-brainer. China didn't have the amphibious capability to sustain an invasion of Taiwan, and the US 7th Fleet could take out the PLAN in its sleep.
Now...probably still would wind up using a combination of naval and airpower (and diplomacy) to bounce the PRC outta Taiwan, but it'd be a tougher fight. At some point (with increasing Chinese naval power and land-based airpower), the cost becomes high enough that nukes start to look like a viable option.
Remember, the whole justification given for the two times nukes have been used as a weapon was because it was cheaper and easier than a conventional response.
Conventional wisdom would say that Taiwan isn't worth WWIII. But Trump is anything but conventional. (Honestly, I think he'd sell Taiwan out in a heartbeat. But then again, maybe he got some shitty Peking duck the night before, so he decides to nuke Beijing. Who knows?)