As the github user mimi89999 pointed, moxie wouldn't have to support LibreSignal directly, it behaves like a normal Signal client and there would probably be no impact in his servers at all because there was a very limited number of LibreSignal users.
Ah! But what if the non-official client causes a DoS of the server....
... then write server software that isn't vulnerable.
I have to agree... mandating what client software is used is a bit disappointing from moxie... I thought the world learned this lesson from Pidgin etc.
Next he'll say he only wants to support Chrome for their website.