They (and anyone else) can access that information if they know/guess/brute force the pin. It can also be accessed it by pulling it out of SGX either through th

They (and anyone else) can access that information if they know/guess/brute force the pin. It can also be accessed it by pulling it out of SGX either through the use of an exploit (There are many many documented examples of successfully attacks on SGX, including examples which specifically targeted Signal's data) or though the use of a built-in backdoor (held by either Intel or the government) as well by other side channel attacks on the cloud servers themselves. When Signal's rule was "We don't collect and store data period" no one had to worry about any of that. That's no longer the case.