"You'll need to prove you own your apps by providing your app package name and app signing keys."
That is capital-I Insane.
The only credible explanation I can come up with is that they need the keys in order to produce indistinguishably backdoored versions of applications, handy for tools like signal.
Otherwise one would never think of requesting the private keys-- if google wants to rebuild apps themselves they could sign with their own keys and possessing anyone elses private key is just pure liability as if there is any discovered abuse they can't show that they weren't the vector.