Fair enough. But even a blockade would rightly be viewed as an act of war, so the ramifications of doing so would be similar to those of an all-out invasion. Even if they do invade, a blockade is likely to be the precursor.
Yes a blockade absolutely is a war act. But it is more difficult to defend against because there is no risk to Chinese forces trying to do some sort of D-Day.
A siege is just about the oldest military tactic there is, and just about the most effective.
In the end my guess is that Taiwan sees the USA can’t help them, and just does the deal.