No it's not - reproducibility doesn't defend you against malicious repository at all. You still need to trust every single maintainer of your packages and the repository owner to not serve you backdoored package.

Yes and no. Just having reproducibility does not automatically prevent bad things from happening. But it's certainly easier to defend against them.

Multiple people can grab the sources from the developer, review and apply the patches, build the package and publish the resulting hash. With reproducible builds, all people end up with the same hash, which should also be the same for the pre-built package in the repository.

In other words, instead of trusting one single person (the maintainer) you split the trust across multiple people. This is definitely an improvement thanks to reproducible builds.