> Content-addressed backups sound something like how git stores data, is that the best way to think about them?

I think it is a valid way to consider them. Another option is to think of the backup as a special kind of file system snapshot that manifests itself as real files as opposed to data on a block device.

> And if so, what would be the main differences between just committing to a git repo for example?

The main difference is that good backup tools allow you to delete backups and free up the space whereas git is not really designed for this.