> How is no marking better than some marking?

With no marking it is clear what it means: commits have not been audited to identify security-relevant ones.

With partial, incomplete marking, unmarked commits can be one of two things: commits that have not been looked at, and commits that have been looked at and are believed to contain no security relevant changes.

The majority of commits will be in the "not looked at" category. And there's enough people around to have a significant subset of them be lazy, ignorant, unskilled or stupid and take that as "contains no security relevant changes."

P.S.: also, patches are already marked. By being included in the LTS series. Because that means they were important enough to get a backport — though not necessarily due to security impact.