There is a large enough proportion of in-service Y-20s that have the flag, that I don’t think that logic holds.
Would the norm not be that operational Y-20s do not have the flag, except for the non-insignificant number that do, which must be for a specific reason (likely foreign “goodwill” or diplomatic ops, like many have posited).
The majority of recent new build/newly commissioned Y-20s and YY-20s lack the flag, so I think the logic definitely does hold.
To be clear, my point is that at this stage it's too early for us to call the Y-20B as being in "PLAAF markings" because that implies we believe it is an in service aircraft.
At best, we can say that it *might* be a a Y-20B that is in PLAAF service which they've chosen to paint the flag on the tail for some reason -- BUT its appearance is also entirely consistent with what we would expect a painted Y-20B prototype to look like. After all, past Y-20 prototypes have had similar appearances and more importantly they've had the flag on the tail as well.
The only way to definitively confirm if this is an in service Y-20B is by having sufficiently clear pictures with serial numbers.
However as it stands right now, we cannot in good honesty say that it is a PLAAF Y-20B, because it could just very well be a painted Y-20B prototype as well.