According to Apple, Safari history and bookmarks are end-to-end encrypted and Apple cannot read them:

https://support.apple.com/en-us/HT202303

Is there any evidence that Apple is lying, and is actually decrypting them somehow and using them for advertising purposes, rather than for the "coherence" feature?

I don't actually want browsing sessions to be unified across devices, so I do wish that it was turned off by default. (Though being able to close tabs remotely is a sort of interesting capability.) However, it doesn't seem to be actual spyware used for advertising or tracking purposes.

That’s not what your link says. The link says browsing history is end-to-end encrypted, but Bookmarks are not.