I've always thought that, considering that it's just an alien VDP in the MSX universe.
It is now confirmed by Kay Nishi himself:
https://twitter.com/nishikazuhiko/status/1481944938564952064
"If your community is committed to 9990 video, That is not MSX and even threatening me to protect your vested interested of 9990 games, I am crystal clear that we are not going to support that chip. You should search the history how 9990 was born and rejected.
Yamaha fail to deliver 9978(later 9990) in time for MSX3. That was the reason name changed to Turbo R. We had only fast CPU. Then Pana used delayed Video chip without our consent saying extra fanctions will be machine depanding non MSX functions.
then by the side, yamaha gave the knowhows of video chip to nintendo and created super famicom without telling us anything. Was that a fair business practices? We dont think so. So we decided not to use 9990 for ever."