I did read the whole article, and I don’t feel like that excerpt addresses it at all. He says “being questioned about speech”, but what that questioning entailed or whether he was allowed to leave (in which case, that’s not actually detainment) is never addressed.

(Aside to the detainment question: e.g. libraries are government-funded; I can still be kicked out of them if I start yelling in one. The Kennedy Center is a public–private partnership, but it’s not like rules still don’t apply if I walked into a fully federally owned government building. I think his argument that he can’t be kicked out of the theater temporarily for loudly booing before the show because the Center receives federal funding is nonsense, even though I wholeheartedly support his booing.)