They're pretty critical of XMPP but aren't there direct counter examples to it, like Conversations https://conversations.im for one.

Conversations is the only counter example and only came into the scene fairly recently. Before, XMPP on Android (my OS) was not fun and battery draining.

Fortunately, as the code is under a free license, F-Droid or anyone else can ship their own builds no matter what Open Whisper Systems prefer.

But their users won't be able to talk to signal users. It is hard enough trying to persuade people to use signal. Getting critical mass for a fork would be even more difficult.

But there's a catch: They may not name it "Signal", probably.

Firefox does the same thing, and I can see the reasoning behind it.