Yeah, TT's ability to guard the perimeter is mostly a fiction and is being wrongfully recinforced by his fairly good ability to stay with the player that he is switched on to, so when we judge his ability to guard the perimeter we usually don't hold it against him when he gets scored on, because if he stayed with his man then he probably did everything he could.
While TT can stay with his man better than most centers, he still lacks the typical size for a center, whether his man takes him to the rim or attempts a shot over him on the perimeter, his inability to aggresively contest the shot is a big negative. This, coupled with the fact that his timing and anticipation are almost always off makes it easy for the perimeter players today to score on him.. players are too good of a shooters nowadays.
Zizic otoh, atleast to me projects to be a a better peimeter defender.. he may not move his feet as quick laterally, but his size and his straight line speed allows him to contest better on the perimeter and at the same time recover quicker to challenge at the rim.
What's the point in staying with your man if you are not going to challenge at the rim effectively? I have seen it too many times with TT.