Thank you, I wasn't aware that they used the term "signature". Before I have only seen this term as "digital signature" from pub-key cryptography, so I think it's confusing to repeat the term for different things (unless there's some hidden relation that I didn't catch). I mean, it was confusing for me, at least.

(personal opinion): On the other hand, I don't understand why it's called a "signature". Pub-key signs are used to prove integrity of some information and some form of authenticity related to the priv-keys. Block header hashes are related to integrity of immutable info, but are not related, in any way, to authenticity. So I still don't find it obvious, nor that it's appropriate, that it's a "signature".