Why can't they be "threatened with a crowbar" to sign the exploit?
If reviewers can build up a reputation anonymously, that will make it harder to find the human who needs to be crowbarred, but I'm not sure how you prove you are a good reviewer in a way which isn't gameable.
Alternatively, the reviewers could be well known teams in multiple jurisdictions, such that an attacker would need to buy multiple crowbars and multiple plane tickets.
This seems focused on signing binaries / build artifacts.
IMHO it seems like if you have the threat model of "crowbared maintainer forced to insert backdoor" you probably don't trust sources let alone binaries and need to vet your dependency sources and then compile your own binaries from them.
Many open source dependencies will not have a jurisdictionally diverse review team, or any review team at all (single maintainer).
I agree that the threat model should include the threat of untrustworthy source code, because we want the countermeasures to work equally well against backdoors, "bugdoors", and genuine bugs.
I suspect for a lot of projects reproducible builds are themselves a bit of a hurdle and not being verified in the rarer case that they already exist, but the point of reproducible + signed builds as indirect source-signing stands.
"Most of those dependencies represents at least one human being who can be threatened with a crowbar and forced to ship an exploit, which can then infect vast numbers of production applications."