My Huion H640P tablet works in krita, but is not in the list of supported tablets. Is my tablet supposed to be supported amidst that?

We try to support users even with the cheapest and most horrible tablets that one can find. It's just, sometimes it's not possible to help them

The list is more like "all models that we know if they work or not".

In general, Huions should work in Krita on all systems except for Mac. And don't worry about it unless you have problems, and even if you have, if you're on Windows, there are simple solutions that can help.