If this guy can figure this out, anybody with the means and motivation to write such an exploit is already working on t.
Embargo is simply a way to make sure the huge, rich cloud providers don't have their reputation tarnished at the expense of everyone else. "Stay with bigco, we fix things before everyone finds about it"
I understand (and agree) that the system admins/owners should also be able to mitigate through knowledge, but it's a dilemma that I think is better resolved by the other solution
(in this case it's apparently a complex issue, but history has shown that there are surprisingly easy to exploit bugs/issues (see heartbleed, shellshock (which was apparently very quickly exploited.))
This guy is not releasing an exploit implementation, he is just pointing to the existence of a potential exploit that has a patch in development. He can't even figure out exactly what it is.
The only people who would be able to code this exploit would be the ones who already figured it out before this guy.