I think it is valid to question why he'd be giving away 99% of his fortune, because let's be honest, Zuck has not proven that he is trustworthy. But at the same time, he could just... Not donate that much.
Yes, the 99% did NOT go straight into non-profits, instead being funneled into his foundation, which has donated millions into actual charitable organizations, but that's arguably millions that wouldn't have otherwise gone to those orgs.
Is it a bit disingenuous to say he's donating 99% of his wealth when his foundation has only donated a few hundred million (or few billion?), which is a single percent of his wealth? Yeah, probably. But a few billion is more than zero, and is undeniably helpful to those organizations.
The retort was essentially "Can't you just be nice?" but people have the right to ask questions; sometimes the questions reveal much corruption that actually does go on