I ran a test with my own channel on Peertube, lemmy, and my soapbox instance. I subscribed to my peertube channel with the lemmy and soapbox accounts, and then posted a message using lemmy. The message posted successfully, but did not appear on peertube or soapbox, but when I viewed the community using another lemmy account on the same lemmy instance I did see the posts. Next, I tried mentioning the peertube channel using soapbox. I knew that post would survive on that instance, but it also didn’t show up on peertube or lemmy.
My conclusion is that if you make a post on a community using lemmy, it will survive on the instance you’re on, but it will not be sent to the peertube instance and will not be federated to any other followers.
Madbrad200