Matrix federation is hard because they want to support large IRV style rooms with consistent history and strong controls to prevent room takeovers.

Signal is more focused on 1:1 or small group chats. Federation there can be much simpler, more like email/xmpp than IRC.

It does still add friction and slows down youe development as upgrading is difficult.

Signal also has a grand total of 40 employees. Keeping the application running on the various platforms takes a decent chunk of their time. Difficult development to add something like federation cannot be expected to happen quickly.