interestingly, MDN web docs claims at the top of the Web Serial page (https://developer.mozilla.org/en-US/docs/Web/API/Web_Serial_...) that Chrome does not support it, even though the support table at the bottom shows that it supports all of the features (Firefox doesn't) and has for longer than Firefox

That's because Chrome on Android has a partial implementation of Web Serial. The banner on top is to get information at a glance vs the detailed breakdown of the compatibility table.