Porting complex applications (especially those written in C) from x86 to ARM can be pretty non-trivial.
I can't speak to the GIMP code in particular, but I've done several x86->ARM source ports of other complex software, and each time there are things which work on x86 and don't work on ARM, resulting in nasty crashes, or worse.
GIMP already supported ARM though, this is about "Apple Silicon". Since the post itself doesn't mention specific challenges, I guess it serves more to inform macOS users of the availability than foster discussion or provide any insight.
I can't speak to the GIMP code in particular, but I've done several x86->ARM source ports of other complex software, and each time there are things which work on x86 and don't work on ARM, resulting in nasty crashes, or worse.
See here for one of those (not AArch64, but ARM nonetheless): https://news.ycombinator.com/item?id=22176285