Interesting.

Ukraine is known for thorough corruption at many levels of government. Zelenskyy first got elected for that reason- he was originally an actor who starred in a movie about a politician who decided to end corruption. He then ran for office on that platform and won. But when the system is thoroughly corrupt through and through, fixing it is not an overnight process.

My initial thought is perhaps this anti-corruption unit is itself corrupt, so it's making up an excuse to justify its own existence.

As for high profile corruption being swept under the rug- what I know of Zelenskyy suggests he would be less likely to turn a blind eye to that. I could of course be wrong though. And I suspect if he brings a high profile treason corruption case against a few well known people and has them jailed or shot for treason, that will send a very strong message to the rest that this isn't tolerated anymore.
That will of course put a target on Zelenskyy's back, but that's always been the case (especially since Russia invaded).