This is an important consideration. Not only was it possible to hide these payments from authorities at the time, they were not even discovered when the club came under heightened scrutiny because Russian owned assets were sanctioned. It's clear therefore that such payments can be hidden relatively easily.
This means there is no way to be sure that other clubs have not engaged in the same activity. Indeed, given the competitive nature of the football industry and the insatiable greed of agents, I'm guessing that many, many, clubs are guilty too. If authorities can't find these payments, but are determined to stop them, it would be counterproductive to impose heavy sporting penalties. Doing that would only make clubs all the more determined to keep their mouths shut about their own regulatory breaches.
There must be penalties but I'm guessing that these are much more likely to be financial than they are to be sporting.