I don't know what splitting up a branch means.
Merge commits 'lock' you in to the commits you've made so far.
I don't doubt that Linus has good reasons for all this. I just don't know what they are. And I don't know if they're applicable to other repos.
If I used rebase like this regularly it would become difficult to determine if a given commit is an ancestor of HEAD. Sometimes I like to do that.
I wouldn't do so if the history were a mess
Looking through my work project git log, it's a sea of random ticket numbers followed by absolutely nothing helpful or descriptive, usually just the title of the ticket if you're lucky.
I certainly have used commit messages and seen others do the same. Perhaps this is more an indictment of the quality of the commit messages than anything else.
In my experience, rebase and squash makes it easier to collect work into meaningful groups and thus write more helpful/informative commit messages.
I can think of a few times off the top of my head when I referred back to a detailed commit message in a repo to understand why a change was made.