This is an addon to protoc that is necessary in order to run the script in the next diff. It's not usually packaged as an NPM package - context here.
The protoc in my local install has this addon already, which is why I didn't seem to need this change in my previous work in D7306. But the protoc in Nix doesn't seem to have this for some reason.
We could probably bring this in via Nix instead. I'm open to that if people prefer.