I asked in that issue whether Lemmy finds community via to/cc (it does). Does PieFed do the same?
Yes - PieFed does the same. It looks in 'audience', then 'cc', then 'to'. It has to, to support all the platforms that haven't adopted 'audience. It's a convenient field, but PieFed won't be affected if Lemmy goes through with removing it.
Would this also open up the possibility of a topic/context being part of multiple audiences/communities?
Not at present. If you do something like cc: [community1, community2] it will only go to community1 (on both Lemmy and PieFed). There's so many activities that are effectively duplicates, both in normal operation and when platforms are bugged (both Lemmy and Mastodon have gone through phases of sending the same activity multiple times), that you need a way to make sure you're only processing one. On PieFed, this is done by having a UNIQUE constraint of the 'ap_id' column of the Post table (the ap_id of your post is https://community.nodebb.org/post/103806), so it means you can't have the same post in more than one community.