I worked in the FOSS space for roughly half a decade. Comments like this are easy to make and also add absolutely no value whatsoever. If you actually feel strongly about it, do the work yourself, no one is stopping you.
They are doing work, they're advocating for what they believe in. Consumers of FOSS deserve to have a voice too.
I have no qualms with him deciding to step away from developing Zulip or setting up a foundation. My qualms are with his choice to work for an AI company when someone of his experience could easily have found a job working somewhere else. Public figures should be subject to criticism of their ethical choices when they make bad ones.