Earlier quoted context omitted.
It suggests he may be Buddhist. You can't really use a religion other than Christianity if you're pandering to Americans.
I see this as baby steps... he knows that being an atheist is a non-starter, politically. Better something than nothing. Watch him get more religious and move towards Judaism/Christianity as we get closer to 2020 :)
That said, there are some religions that are compatible with atheism, which wouldn't necessarily be recognized by the public as a religion that could have atheists in it, or would at least provide plausible deniability if you are smart enough to keep your mouth shut about religion on the campaign trail.
Unitarianism, obviously, is one of these. If I were ever to run for office, I would simply join a UU assembly and leave it at that. This would probably restrict me to running as a Democrat, though. If I wanted to run as a Republican, I would have to be more careful, and say "I was baptized in the Methodist church".
If Zuckerberg wants to give it a go as a Buddhist, secular Buddhism exists, and certainly no one on the west coast would bat an eye at it, but the other regions of the country would probably either have a problem with the religion itself, or that Zuckerberg would be practicing it as a white guy.