So apparently this is what's going on, put in words as simple as possible, but take it with a grain of salt because I'm no expert neither:
The story about Juventus and the infamous plusvalenze originally went like this: Juve traded players with other teams at prices that were objectively much higher than their actual realistic market value in order to whitewash their own accounts. The best-known examples, who were anything but hidden back in the day: Pjanic-Arthur, Danilo-Caicedo, Caldara-Bonnucci (and many more).
In May, the competent authority decided that there was no such thing as an "objective" market value, that the market determined the price and that Juve therefore could not have valued the players too high. The case was thus closed.
Last September, however, the FIGC filed a motion to reopen the case because - and this is important - NEW information came to light. Namely, that Juve not only valued the exchanged players higher than was "objectively" realistic, but that they even gave the exchanged players HIGHER VALUES in their books than they had AGREED with the other teams. THIS is the offence that FIGC accuses Juve of and for which Juve was punished with the deduction of 15 points.
What happens next? Juve will appeal within 30 days. The competent authority will in turn have 60 days to uphold or reject the appeal. Either way, it will probably take three months (30+60 days) before we know for sure whether the point reduction will stand.
The point is this: Juve argues that they were punished twice for the same crime - which is of course not legally permissible. The FIGC, on the other hand, argues that there is new information on the table, which changes the situation and thus justifies reopening the case. Juve, on the other hand, replies that this is not the case and that there is no "new" information - they are not aware of any guilt. Only time will tell who is right. No information is available to the public. We are left with the battle of words between the parties.