> Isn't this also the case with Matrix that no implementations except the official ones support E2E encryption?

This isn't true. There are independent working E2E implementations in weechat-matrix and pantalaimon (python-nio), nheko (mtxclient), and even a read-only one in purple-matrix. Meanwhile lots of independent apps build on the official SDKs (e.g. Seaglass on macOS, bots like Matrix-Recorder, the various Riot forks, etc)