I think you’re justified in feeling a little off about discovering the app still on his phone. This is something I see a lot on different relationship subs; people are always upset that a partner kept a dating app on their phone. I never used a dating app (I don’t think they were a thing when I met my partner) but I can understand how finding out a long term exclusive partner having one can be upsetting.
But he wasn’t using it. He was still logged in as that fake profile, you saw or yourself, so I supposed even if he did turn the app on again after the time you used it to find your sister, it wasn’t even him. He’s probably being honest that he just never deleted it and it just ended up moved into that section to make space on his screen.
Just let it go. No harm came of it. It’s okay to be upset and people fight about stuff... just let this one go, it’s not worth worrying about. Nothing happened.