But why would that be the case
Any number of reasons -- having additional cargo bay fuel tanks may require additional plumbing in the aircraft, it may require more time to load an additional tank which as a more regularly configured MRTT they may deem to be too time consuming,
has there been any evidence to that effect? IIRC from the YY-20A's photo we could see one tube running upward to the wings and another running straight downward and backward (through a pump mechanism) to the central drogue port. Without that central position & pump on the Y-20B if anything it seems like you should expect it to have more room for the internal tank, the cargo bay dimensions otherwise being identical.
I think you're reading too much into the post which you quoted from me.
I wasn't saying what I thought
was likely to be the case -- I was answering a hypothetical on a hypothetical to rationalize why it may be possible that a YY-20A may still have some uses as a tanker relative to a tanker fitted Y-20B, and in #6884 I did explicitly write that of the two hypothetical reasons I listed, the more probable reason for continuing to use YY-20A alongside Y-20B was its central fuselage drogue station, and the cargo fuel capacity of YY-20A vs Y-20B was more of an unknown hypothetical.
Thanks so to sum up we can conclude it is MRTT from the fact that it has refueling formation lines while not having refueling pods currently installed. I wouldn't phrase it like that. The problem is that you are entering this topic ("Y-20Bs are MRTTs?") part way through, so you are asking the...
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Obviously as of present we have no idea what the internal cargo fuel tank for Y-20B situation would be as of present. It might be able to carry the same/less/more internal cargo fuel than a YY-20A. In theory they're all equally possible at this stage.