Jablo's article above made a solid answer about this. Specifically, that it's Porzingis's 1v1 defense vs other offensively skilled bigs, where he can't just sag off the weakest offensive player to be the help guy at the rim (which is our default stance until the guy in question makes them pay). And if we don't help when he's 1v1 with the likes of Jokic or Embiid or AD, they tend to run him over, because his center of gravity is too high and he doesn't have enough strength to hold those guys back. Jokic, of course, has an even-more-deadly toolkit to work with.
https://lowmanhelp.substack.com/p/the-boston-celtics-are-the-death
They also noted how he's been attacked by the other elite teams, beyond just Denver, particularly Milwaukee, Minnesota and Oklahoma City. I think we saw last night that Chet Holmgren isn't going to exactly dominate vs Porzingis 1v1, especially with how often we'll send help because he won't find the open man as readily (he did get a few over on KP, including that reverse dunk, but to my eyes KP largely won the matchup). Minnesota and Milwaukee have bigs who are great 3-point shooters that you can't sag off of (Towns and Lopez), but in practice we don't put Porzingis on them in a half-court set defense, we have Horford or a wing on them, with KP on whoever the weakest outside shooter is. So to attack this weak point, you really need to have a combination of:
1) A big who is a massive threat in the post and will run Porzingis over without help
2) ...who can preferably also shoot from outside or at least be a great passer when the help comes
3) A starting 5 of all-shooters so you can punish our default positioning with Porzingis as the #1 help defender
4) ...who then aren't giving up even more on the defensive end (Dame, arguably Porter)
I think Minnesota fails 2 and 3 (Gobert is only a paint player on offense, and if we help on his post possessions, any of McDaniels, NAW and Kyle Anderson are options to leave open for a moment). Milwaukee can't exploit #1 because we won't usually have Porzingis on Giannis (and Lopez can't run him over), and probably fails 3 if Middleton can't rediscover a higher gear (or if Pat Bev or even Crowder are on the floor), with defensive issues also accompanying it for #4 (Dame, however...*). The Clippers basically don't have #1 (Zubac is not winning that matchup), though they present a very different problems in terms of how you help. The Lakers and Pelicans aren't winning the West. Myles Turner has a lot of agility advantage on us, but we have edges in many other places on the floor. So it's really just Denver that has what it takes to exploit that advantage.