In that Beato video above, Rick says in passing, "Theory is just naming things," and I think that 5-word sentence pretty much sums it up.
My mother is one of the best pianists I've ever heard. She was the pianist for whatever church she attended from age 14 until well into her 80's. For decades, strangers would come up to her after a Sunday service and say, "That was the most beautiful offertory I've ever heard!" It happened all the time. She'd be visiting with family or friends after a church service, and I'd see a stranger a few steps away... just sort of waiting their turn to talk to her, and I could tell from their body language what they were waiting to say.
She can't read music... except looking at the top left of the clef and counting sharps or flats. She knows from the symbols what the root chord is - and that is all she needs to know. When I was a kid, she might say to me after a service "I just hate when (musical director) starts singing that song in four sharps!" I always thought that was a funny thing to say. She would never say, "I don't like playing in E."
I never heard her practice. She'd occasionally go to the piano before we left for church on Sunday morning, and she'd play the offertory one time beginning to end -- flawlessly. But as far as hitting a bunch of wrong notes until she could figure it out... or struggling with a difficult passage over and over again until she got it right... I never heard that. Not once.
Anyway, I don't know if it would be fair to say "She doesn't know musical theory." She may not be able to articulate what she is doing, but (like Chad says) she absolutely can speak the language. Theory is just naming things.