is there a flavor reason this should be hosted as x68k? if it's denoted as ym2151 only we have a format for that, so it should probably be that format unless there is like, a story reason or something 🤔

Yes. Because the instruments are from a x68k game. There are games on the system that only use the fm chip.

i still think its weird to host a format and then disallow that one thing that makes it different from another format

This is the x68k format, not the ym2151 + OKI MSM6258 format.

Similarly, there are sgen games that don't use sampled drums. They only use fm drums. But at the end of the day, they're still sgen games, with sgen soundchips playing on sgen hardware.

If these ym2151 instruments belonged to a non-x68k game, I'd have hosted them with the ym2151 format.