I would say this is a pretty good example of an ad hominem attack.

https://en.m.wikipedia.org/wiki/Ad_hominem

If the standard is that someone has to be unimpeachable before they can suggest a well-being metric than no one can.

I disagree. When one demonstrates contempt for the public we should be inclined to pay less attention to their opinions on public welfare.

No this is an example of pointing out hypocrisy. He says that others should do what he is not willing to do. If he were to practice what he preached, the bottom 50% of income earners would have access to his beach.

The messenger doesn't need to stand by his message

Two things can be true: The message is good, and the messenger is also hypocritical.

Technically, there was no ad hominem fallacy. RcouF1uZ4gsC just argued that Khosla is a hypocrite.

Agreed, but under a charitable interpretation.

Under a less charitable one, RcouF1uZ4gsC was discrediting the message by using the messenger's ``ethos''.