Similarly, however that whitelist of identities is specified becomes another vector, without a lot of PKI
My goal is for readers to match the hash to one on Github so they know the code they're reading there to vet the package is what's on npm. For OSS maintainers, it's an immediate red flag to see a package version published with a hash that was never in the public repo.
This won't certify that the version is blessed by a legitimate author. It just delegates the problem to Github (or their competitors). It doesn't solve the problem of preventing all OSS library-based attacks, but plugs one of the leaks in trust that caused today's failure: that Github->NPM is not checked in any way.
Code signing still allows the fire to spread. What we need to be talking about is how to prevent the spark that starts a fire.
Otherwise when vetting an npm package, you have no idea if the code you're reading on Github is actually what you're going to download or not. In scenarios like this one, package publishers could easily configure getting an alert that someone tried to push to npm a hash for which no commit exists in Github.
For packages with a build step this would likely mean npm would have to run the build step on their servers. Hopefully there's a way they won't have to eat that cost, but I believe publishing linking package versions to specific git hashes is the only way to prevent an attack like this.