Not a real spoiler because they may not follow the book. It’s also slightly ambiguous in the book. In the book there is no Brienne/ Hound fight. I guess they decided the Hound deserved (at least) one last good fight.
As to people who think Brienne could not beat the Hound, no basis for that. They are two unstoppable characters.
My justification is the Hound could easily block his brother’s swings during the 1st season episode. I don’t see Brienne being able to stand up like that to the Mountain.