You're right, brown trout are very adaptable. Also, that little brown that fella is holding appears to be a wild fish. Brown trout are in lots and lots of places.
Just a quick aside/FYI: The guy with the handle "mike" is a retired PFBC biologist. He definitely has a lot of knowledge on wild fish, where they turn up, etc.
I don't doubt that there might be some wild brown trout there. If there are, then why stock it? The fish are there. Go catch em. Maybe improve the habitat if you can to help them.
Spending resources and time for stockies if a population of browns already exists seems silly, though. I'd report the findings to the PFBC so they can come do a survey and check the biomass levels.