Having read what tebas said, or the reported version, I think it can be interpreted two ways.
Barcelona are above a wages over cap threshold which triggers a sanction that means they can only spend 25% of what they raise, in both transfer fees and wages.
So if with messi on their wages they are over the cap, messi is removed from the wages they fall below, that imposition is removed and he or anyone can sign for them without having to offset funding 75% of that by sales or wage reductions. Sounds perverse, and the timing might not really work, but purely reading what he said, it is there, for me.
The other side is, as you suggest, that now that he hasn't re-signed as a continuing player, he is effectively a new signing and falls under those same restrictions, assuming they are still over the wage cap threshold. I.e if they are to pay him 50m a year, they have to free up 200m to do so. Making it impossible to sign him. Which seems the more logical one to me tbh.
Tebas obviously has the say, and if there are two interpretations to choose from, doubt it will be one that has an adverse effect on Barcelona.