I wouldnt call it PIS, given that the only time he didnt was the fnal battle, where he only had one and pure hearts were a thing. Every other time he kinda just, let mario go so he could gather the pure hearts and defeat Bleck for him.
Pretty sure he teleports the sprout on others, he doesnt manually plant. given he did it to Mario without actually touching him, only snapping in the bad ending. Count Bleck had the Chaos Heart and had minor invincibility which is a lower version of Dimentio's own, which allegedly was supposed to make himself completely invincible, which sould at least extend to what Dimentio is aware existed given he said there was nothing the cast could do to do anything to him, probably made Bleck untouchable as well. Then again, Dimentio didnt want Bleck defeated right away, he wanted Bleck defeated after he kickstarted the void, he even helped Bleck a bit, honestly Dimentio's plan was kinda convoluted.
The invincibility is passive, unlike Bleck's, which was a forcefield. Although if X has the higher AP then it'd NLF to say he cant harm Dimentio given the pure hearts could negate it despte only being 2-B on the equal terms with the chaos heart, if its lower though then it stands though, although I have no clue which is higher so you all can decide on that.
Do note I'm not arguing for Dimentio or X here, I'm just saying what Dimentio can or cant do.