Your boundaries are for you to maintain. Someone doing something in private that doesn't involve you doesn't mean they're violating your boundaries. No one owes you their privacy. You can't expect them to be open about it if your first response is to be all hostile to them.
The reason your friends have "secret proship accounts" is because they don't feel safe enough around you to tell you. They're literally afraid of what you'll do if you found out.
And OH BOY y'all will never believe WHY that is
The thing is, if you're that worried about being around proshippers, you might even be interacting with one in real life. And no, they're not on twitter or in any fandoms but they hold the same belief that fiction is separate from reality.
I don't label myself as a proshipper anymore because I believe that being able to separate fiction from reality and being anti harassment over art is literally just being normal. There are a lot of ppl out there that don't like labelling themselves but hold the same stance.
You can be edgy all you want by saying that you're gonna make secret proshippers feel "unsafe" around you, but we all know that moment of bravery doesn't really extend offline, unless you're a completely unhinged type of person.