If ISP doesn't matter, then don't make it a rule. Wasting everyone's time.

" he just outright wasn't there from the start "

That is the definition of ISP.

"would've taken a suicide had you voted anyone else."

If I voted "no one", or someone else,
it would still be a KL. He would still be lynched.

What do you not understand about this?

You didn't "have to KL" you chose to do it instead of vegging the game. Insufferable didn't ISP, he just outright wasn't there from the start and would've taken a suicide had you voted anyone else. Given that they have no history of GRS/Repetitive suicide/veg, there is no reason to believe this is a violation.