It's great to see them making the effort, but why not notify the OpenPGP.js with the outcome of a security evaluation? (I'm not aware of any other active javascript openpgp implementation, so I assume that's what you're referring to.) I've been following OpenPGP.js for a while and I've not seen anything from Google.
OpenPGP.js is not an amazing code base, I know this for sure. Perhaps a rewrite was the only way to salvage it.
I wonder why they didn't release the library independent from a browser extension. A brief look at the directory structure makes it seem that it wouldn't be too hard to decouple the OpenPGP implementation from the extension.
In any case, this is a big win for privacy. Reinventing the wheel or not.
Unless I'm mistaken, the author appears to be implementing OpenPGP in javascript. This has already been done by OpenPGP.js. That project is several years old, is active, and has been independently audited.
Is this simply reinventing the wheel? OpenPGP.js can easily be used in an arbitrary browser extension.
I have no affiliation with the OpenPGP.js project besides working on a small project for personal use.