But they don't have to use the same client? The web, iOS/macOS & Android SDKs are completely independent, and have different clients written on them. Riot may be the main one, but Seaglass (on the macOS SDK) has full E2E support, and there are loads of projects building on the matrix-js-sdk which inherit its E2E support. Meanwhile the nheko project has an independent from-scratch implementation of the E2E stack, Fractal is adding it too (thanks to funding from Purism), and even Pidgin has it (albeit read-only currently). (N.B. that the reddit post gets almost all of this wrong :|)
So yes: Moxie has a point that the more implementations you have, the more bugs and security holes you may have, and the slower the project can evolve. But for us, freedom to control your own data and conversations and provide an open network & platform to build on is more important.
Sure, not exactly the same client, but at least you should make sure that none of the participants use one of the long tail of non-mainstream clients. Being able to do so would be one of the main selling points of decentralisation for me.
So yes: Moxie has a point that the more implementations you have, the more bugs and security holes you may have, and the slower the project can evolve. But for us, freedom to control your own data and conversations and provide an open network & platform to build on is more important.