Yes I read that and was surprised considering the IIx debuted with 6.0.1. Interesting ... I wonder what the physical difference on the logicboard was that caused that incompatibility since the IIx is often called simply a II with a 68030.
Anyway, I do see there is a physical incompatibility ... while the ROMs of the II, IIx, SE/30, II FDHD, SE FDHD, IIcx, IIci, IIfx & IIsi are all software interchangeable, they are not the same same physical size. The SE FDHD and II FDHD are in essence IIx ROMs in a different physical configuration. I assume the IIx, SE/30, IIcx, IIci, IIfx & IIsi all use the exact same ROM SIMMs, whereas all the others use custom chips. Perhaps this is the reason the II FDHD requires 6.0.2, due to the different pin configuration from the SIMM to the chip? That or there are more differences between the II FDHD ROM software and that in the IIx ROM.
So I also see, no one was able to ever use a IIfx ROM on a stock Mac II. I have to therefore use the transitive property of equality to assume if a Mac II can use a IIx ROM, and a IIx can use a IIfx ROM, then the Mac II can use a IIfx ROM. This is further supported by the fact that the II has a 68020 CPU, but can use the ROM of that for a 68030 processor with no ill effects. Unless of course, there is more to the Mac II FDHD ROM than meets the eye.