It looks like Signal isn't as open source as you thought it was anymore

It's been like this for a while, and the project owner's attitude is pretty negative overall. I do use signal daily, but I believe it's likely compromised ala lavabit.

What's in the Signal server to be compromised?

List of phone numbers? Pairs of communication partners? Timing and size of messages? Metadata about transferred media? There is still a lot, sufficient for targeting a drone strike as the usual wisdom goes.

Some of that information you don't even need a backdoor to collect; the rest is stored in plaintext by Signal's competitors.

Signal claims to specially protect some of that data, such claims need verification. Storing or not storing that data needs verification, without the trust that they do what they say they are no better than their competition. Trust is earned e.g. by openness about the source code. And that a server backdoor isn't strictly necessary is also beside the point because the server is the easiest and most obvious way to get at all that data.

Also, there is competition like Briar which has less of those pesky metadata problems (but some other problems instead)

I don't recall Signal ever having made implausible claims about traffic analytic attacks. I also don't buy into the idea that platforms are as trustworthy as their source release policies are orthodox.