Have you ever filled a tax form before? Because you sound naive. Everyone tries to reduce taxes through deductions. Tax rules by definition are vague and open to interpretation because of the number of different cases they need to work in. It's not a black and white case of either being 'exposed' or doing 'nothing wrong'. When your revenue is as complicated as Messi's is, there is no 100% 'correct' tax declaration, the only thing you can do is to make sure you follow the laws and don't intentionally try to cheat. Now, given that Messi has been caught cheating before, any small bits that the tax agency can argue against by virtue of things being as complicated and subjective as they are, I'd reckon they'd look to dig it open. This doesn't mean his accountants did anything 'wrong' or creatively 'avoided' tax, at least not necessarily. It just comes with the territory of being in the public eye and having been caught before.
Millions of people around the world get tax notices for a deficit in tax payment or a tax asset. That's just how taxes work.