If everyone stands around at low health, Kat is gonna get a penta, and that penta does count. Saying it doesn't is just a classic example of salt and No True Scottsman fallacy.
The face of truth.
Sodium talks
Unfortunately, the truth of the matter is that when you play as Kat or Yi, achieving a penta does not really count. Picking either of those champions is basically telling everyone involved, "I'm taking the Penta Challenge".
This is coming from someone who has achieved a Penta with Eve, Kat, Yi, and Wu and almost as a few other champions including Ahri, Heimerdinger, and Rammus.
That doesn't take away the fact that getting the Penta as Kat or seeing an allied Kat get the Penta is fun. It's quite exciting to see it happen, actually.
It's like that with almost every Assassin champion and a few Marksmen champions too I found out.

get a Penta with any of those Champions and it doesn't count.
the people that say that are just a bunch of babies being babies
Yi and Kat are the Penta King and Queen because they're very good at clean-up, really. They can't 100-0 to a full-health enemy team by themselves unless they're really ahead.