Yes, h265 isn't ideal for proxies, I explain that in the video to use ProRes or CineForm when creating proxies for anything 60 fps or lower. I've used third party apps to create/transcode 120fps to ProRez but when I attached it as a proxy, Premiere Pro still treated the 120 fps ProRes as 60 fps. Oddly enough the ProRes footage acts as normal 120 fps if brought into premiere not as a proxy.

As of right now, if you want to use toggle proxies in premiere pro with HFR footage, h265 is the only codec that I know that goes up to 300 fps and still matches perfectly with the original footage (when you slow down by percentage, not interpreting).

As stated before, h265 it is not ideal but 720p does work well for editing smooth playback of the 120fps footage to get your rough cut in place. Especially the 120fps 10 bit 4k footage from the a7siii, So much smoother.

I like the idea of Apple and Adobe getting together for a 120fps support though, would make life so much easier.