Yes, Thiels can be bright, if not properly setup, with the right gear.
And if you compare them to Paradigm, then they will really seem bright.
I consider Paradigm to be dull compared to my JBL PT800s and I don't consider the JBLs to be bright at all, just clear.
So how a person describes Thiels, JBLs, Paradigm or any other speaker directly depends on what kind of speakers that person is used to listening to.