Anyone can build an implementation of the Signal client, but few do already because Signal actively works to prevent them from working with the Signal infrastructure, and likely will continue to do so. It’s one of the more common complaints about Signal, but it was built on the assumption that centralized services would be easier to use and to make private if the platform holder wanted, as well as more robust against attacks. They could well be wrong, and people just haven’t thought of and deployed the right tech, but it’s neither here nor there; I’m doubtful they can be convinced on this, and I’d doubt they’d be made to open up anyway by this regulation, meaning they’re not obligated to.