The GPL doesn't require them to provide source with any new release of the official client. They're the author, and hold the copyright. They can decide tomorrow

The GPL doesn't require them to provide source with any new release of the official client. They're the author, and hold the copyright. They can decide tomorrow that all future versions of Signal will be closed-source, and then just stop providing source. They can't retroactively un-GPL existing released versions/code.