Lol, it's federated, you don't post "on Pixelfed" you post on whatever host you're on and maybe that host is federated to lively other hosts
I understand that pixelfed is federated, but I still knew exactly what the commenter meant when they said they posted "on pixelfed". They meant they posted on an instance running the pixelfed software.
It seems like it was effective communication to me, and your correction seems both dismissive and unhelpful.
There is no strict technical definition of “on” in this case.