This is a fastly growing messaging startup we're talking about. You may be right, but to me it sounds highly unlikely they haven't had to make even the slighest patch to their server code in 10 months.
Also let’s not forget that Signal was founded 6 years ago so they’ve already had some time to work on their backend code already (and in a sector that isn’t exactly lacking of research papers and prior art for solutions).
I'm aware they're looking at backup features and such, wondering if they'd be part of the server, or a different system.
I'd expect some improvements to performance and such, esp. as they grow, so good catch on the 10 month wait, but it's an orange flag rather than red flag to me.