they just lowered the brightness and removed promotion, these are things that can be done in software/firmare, you don't need new hardware to change brightness and remove pro-motion...I'm wondering if that's exactly what Apple did... Could it be as simple as Apple just taking things out?
Granted, I don't know the technical aspects of which screens they used, but the 2019 Air had 500 nits of brightness also and didn't have ProMotion. It looks like, at least screen technology wise, that the 2020 Air used an upsized version of the 2019 rather than taking the Pro's features out (unless it is that easy).
Maybe someone with better technical knowledge can confirm?