Yea, I've been pondering this possibility as well. If this is the case YT should face considerable backlash from 2019/2020 model year bike owners because they (YT) have consistently used the release of 700W batteries as a selling point, added to which they also said they would discount them to buyers later.
Of course, I'm not sure who you are or how you would know, or if you are just speculating, BUT the logic of getting a larger battery with the E9000 as a kit is sound except YT was one of the few to custom size their batteries for Shimano so the situation is different than other manu's.
On another note, I've spoken to YT USA a few times on the battery and they have not indicated anything other than that the 700 will come out but we have to wait. In one conversation a YT rep said to me that newer Shimano motors yet to be released were going to be backwards compatible in terms of the mounting (on 2019 frames), this was in October/November but I call bullshit because how would they know?
Lots of hearsay going around, wish YT would come on here and clear the air.