I agree -- they chose their platform (docker), and for my personal work that just isn't my platform, and it's probably unfair for me to call them "opinionated" about that (because like it or not Docker is the norm) or about Ubuntu, especially in light of where I was getting my information from.
I've seen some flame in this community, but damn this thread has just been a bunch of polite people helping correct the record.
And yeah, trying to use podman with something that's based on docker compose is ... probably gonna give you some headaches, I'd guess. I don't particularly know the pitfalls but if you're expecting it to be transparently swappable, I don't think that's an owncloud issue.