It's definitely not rubbish although even a Spanish tax expert probably couldn't tell you exactly how it is. Normally when two clubs agree on a fee there's no income tax involved because one club pays another a fee for the rights to a player. That's also what usually happens when one club meets the release clause of some player, they don't really pay the clause they just pay the club the amount stipulated in the clause.
However if the selling club does not want to sell at all like with Bilbao with Martinez or Herrera then even if just for a second they're transferring a substantial sum to the player's personal account so that he can pay it, and some Spanish authorities (some because it wasn't an issue during e.g the Neymar transfer I think) consider that money even if it's lying there for just one day to be taxable income.
Large parts of Spanish law are stuck in the dark ages but in this case it really does make sense to keep the situation ambiguous enough for a case by case distinction. If you ruled all monies paid to meet release clauses to be non taxable income you'd be opening the door to all sorts of tax dodging fuckery.
