The author mentions doing their test in Shelter, which is what I see everyone using for work profiles, unfortunately. Not sure if it could function differently for graphene than stock.
I've been thinking of it more like an inbox selector. I got tripped up by it, too. It would make sense to keep the account and feed tabs in sync.
Testing this out right now. I have a lemmy world account logged in while viewing the post through another instance.
If you're seeing posts from multiple instances, it could be the "everything" tab. In the settings it can be disabled but should be off by default. There's a different All, Local, and Subscriptions view for each signed in account for me.
Each post says via instance.here and my guess is you're commenting with whatever account matches.