That is very vague. Even when Bush did it was very vague. We went into Iraq for "suspicion" of WMD's. There was no immediate threat to the US when they planned it even if there where really WMDs. Threats to "national defense" are often if not always vague.
I could come up with 10 reasons why CCP attack or takeover of Taiwan is a national defense threat. Or at least vague threats like Bush used in Iraq.
- Military bases in Japan more vulnerable.
- Military bases in Korea vulnerable.
- Military bases in rest of Asia vulnerable.
- Maritime Safty.
- Tech Chip Production Security.
- We could have citizens there at the time.
- Protect other allies in the region.
- Lose control of the SCS & China being able to assert its 9dash line.
- Lose the rest of what little left in credibility the US has which has defense complications.
- Possibly is a secret backdoor treaty already in place that ties our hands into coming to Taiwans defense.
Any of these vague reasons are fine enough to militarily physically defend Taiwan by any President not just Biden.