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That's not correct, Libreboot does not work on those systems that require any part of the ME to boot.

You might be thinking of me_cleaner[1], which removes most but not all of the ME blobs. This is unrelated to Libreboot though, it works on newer systems and is not needed when using Libreboot because the latter gets rid of the (very early versions of) the ME completely.

[1] https://github.com/corna/me_cleaner

Are you saying a librebooted machine is still probably backdoored?

Precisely.

You are talking about Coreboot, not Libreboot. The latter has no blobs whatsoever.

Does that mean that a firmware backdoor is impossible or very unlikely?

Yes, it should be harder.