Yep, that’s the gist of it. In order to change the license from the GPL, they’d need the permission of all of the copyright holders who’ve contributed code under the GPL to the project. After a few months have passed, this basically makes it impossible (or at least extremely difficult) since at least one person (and likely many people) will say no.
It’s not perfect yet, but it’s much, much better than the old days.
OMEMO is supported by every major client, and they interoperate successfully. Unfortunately, most clients are stuck with an older version of the OMEMO spec. It’s not ideal, but it doesn’t cause any practical issue, unless you use Kaidan or UWPX, which only support the latest version.
All popular clients and servers support retrieving chat history now too.
In practice, I’ve been using it for several months to chat with friends and family, and haven’t had any issues.