Why should he care about the pledge? Taking the letter at face value, he's already gone far further ("at least 95% of my wealth") than the pledgers, and he's do

Why should he care about the pledge? Taking the letter at face value, he's already gone far further ("at least 95% of my wealth") than the pledgers, and he's done it in private. I can totally understand him not wanting to write such a letter and doing it only as a favour to Buffett, with a certain skepticism. Well-founded skepticism too. (Or maybe he's just pissed that he had to blow his cover as a supervillain.)