Thompson has been able to play championship defense only for a couple of seasons and mostly with the help of lebrons court presence.
"only" is doing a lot of work here
Thompson was part of the defensive group that stymied the 73 win Warriors.
Over the course of multiple deep playoff runs he was able to stay on the floor against some of the most offensively loaded perimeter teams the league has ever seen.
He may not be quite the same player now, since he's a couple years older, but it doesn't make sense to me to think that he wouldn't be able to do similar things defensively anymore. He's only 29.
Not to mention that other than some awesome chasedown blocks LeBron often doesn't do a heck of a lot on defense, he takes the easiest assignment and saves his energy. He did that a lot on those Cleveland teams.