OK, but if compromising SGX is pretty easy, that just leaves the servers (hosted on AWS) for someone to be able to observe and decrypt all Signal traffic, right?

It is anything but easy. I guess that is why you get downvoted. The fact that only this academic group was able to come up with these attacks shows the amount of knowledge you need to have to carry them out.

OK, but how hard is it for a state-backed intelligence agency to come up with that knowledge? The Vault 7 leaks showed that the CIA has all kinds of very deep capabilities in this area. I'm not thinking of some random script kiddies doing this.

Past news should have shown that pretty much nothing is invulnerable from state-backed intelligence services. I would be more worried about what can realistically be hacked by random people on the internet.

it's fairly easy (with the available publications) once you have the prerequisites: a compromised operating system

which is of course exactly what sgx is supposed to defend against, but at the same time that kind of access is a pretty big problem on its own