In what sense, that we literally didn't know how to do e2e across messaging networks? The Signal protocol existed in 2016 when Schiller argued against cross-platform (also PGP for 30+ years). Even granting that, offering iMessage on Android would satisfy most people and doesn't require operating across networks so any argument about feasibility rings very hollow.

https://www.theverge.com/2021/4/27/22406303/imessage-android...