TL;DR: they bridged Elm with React, and when they started using Web Components there was an impedance mismatch between the bridges between Elm/React and WebComponents and the bridge between Elm and React, so they had to pick one of Elm and React.
I liked the description of the internal discussions. Working at a larger company where decisions are made in a completely different part of the organization and you're stuck dealing with the ramifications leaves me a little nostalgic for a smaller firm where it's practical to engage all stakeholders.
It was kind of cool that they really explored all the avenues for making it work.
Because React and TypeScript had progressed since they first decided to use Elm, because the cost of maintaining two frameworks was high, and because the cost to switch to Elm was out of reach, they decided to “contain” Elm by no longer permitting it for new components.