Thanks for this, you answered my questions thoroughly. I'm not entirely convinced by this though:

> [A signed commit] doesn't guarantee anything about the contents of that commit if the repository has been tampered with.

I think most people would intuitively expect the signed commit to guarantee the contents of the tree being signed. The idea that you could "git pull" a repo from a compromised machine, verify the signed commit, but not actually have a guarantee about the tree matching the one that was signed would run counter to most people's expectations, I suspect.

In other words, this to me seems like a "technically, we don't guarantee" statement about something that is de facto thought to be guaranteed.