I'm saying that if we're picking and choosing our seasons (like 2018 for example) it should be noted that he's never had a post-season where he improved on his regular season performance. You can find reasons to explain that away all you like, as is the nature of sports, but it doesn't change the fact.
I would expect that most players, on a per-possession basis, get worse in the playoffs. They're playing against better competition, teams specifically game-plan against them, and guys have just completed a long season.
And, "he's never had a post-season where he improved on his regular season performance" isn't really true. For instance, in 2018 he put up 24 / 6 / 6 / .565 eFG%.
But, if he "only" performs at his career playoff averages of 18 / 7 / 5 with elite defense, I'll take that over either Smart or Brogdon.
Lastly, where did I pick and choose seasons? I cited how impressive he was in 2018, making All-Defense while playing out of position. But, it's not like he hasn't been excellent over the past five years, or really, his entire career. Unfortunately, in two out of three seasons, Marcus didn't consistently show up on defense, and Brogdon's D was terrible last year (despite him calling out his teammates on that end).