Any secure co-signing approach would require putting app developers in the loop. Are app developers going to do that? If developers want to provide device-optimized APKs, they can do that already without this Google-signs-the-APK stuff.
I do know a bit about PKI. And one option might be that the workflow be something like:
Dev signs bundle => Google signs bundle (including dev signature) => User/Client verifies both signatures.
Google are still signing here, but if the dev signature doesn't verify (or isn't present), there's a post-build modification occurring, which is the principle issue raised in the tweet thread.
While APK backups aren't officially supported and therefore might not factor into the decision making, curiously enough Google itself only recently added a feature to the Play Store which apparently allows direct sharing of APKs between nearby devices. Split APKs however mean that you can only sensibly share apps with devices having the same screen density, language, etc. as your own device, somewhat devaluating that feature again.