I was waiting until I saw something show up in my feed to verify and I just saw it.
Peertube is the fediverse version of youtube. Different peertube instances can connect together so people can view videos from all over the Internet from the one user interface.
I was running some testing figuring out what I could federate with, and I found that if I plug the URL from a peertube channel into my lemmy search, it shows up like a community. Then I can follow it and new videos will show up in my lemmy feed. (I expect the same would work in kbin)
So for example, the minetest videos channel is at https://share.tube/c/minetestvideos/videos – Just plug this URL into search, and suddenly minetestvideos is a community you’re following on lemmy or kbin, and new videos will show up in your feed and you can watch them and comment on them right from here!
It’s a really great example of how ActivityPub support lets you connect things you’d never expect to be able to connect. Imagine if on reddit you could just subscribe to a youtube channel!
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.