That's what I was thinking. Red Bull is saying their submission came in under the cap and apparently it didn't take FIA much research to find the issue. To me, that sounds like Red Bull thought they found a loophole or have a different interpretation of a specific budget item(s) (i.e., that it/they don't count towards the cap), and that FIA disagrees. That would mean Red Bull is counting the same way this year, and would have to cut costs quickly to not end up over the cap again. But it probably also means legal action, as Red Bull will have been convinced of their interpretation ahead of their submission and are unlikely to just accept FIA's interpretation and whatever penalty comes with it. (Except if FIA offers a soft penalty as a one-off compromise - which they might if there really is some grey area in the rules.)
In any case, all the reactions in here about Red Bull being fraudsters and FIA being corrupt seem rather premature - but then of course no-one expects nuance from this thread.