Huh? Yes there is, slamming is clearly listed as an illegal technique iin the link YOU posted.
Slamming is illegal ibjjf rules if you slam them from closed guard or a sub attempt.
The only rule for takedown is not slamming on the head from supplex.
Also head on the outside for while belt for a single leg takedown.
But it has been a while since I have done am ibjjf ref seminar.
So is there a written rule I missed or people are just reffering so something else.