Technically, it is not an incremental backup, because the backups do not depend on each other.
In an incremental backup, there is a “base” and each incremental is the difference from some base (or another incremental). This forms a chain of backups, and can break in various ways.
Duplicati is more like a “differential” or “deduplicated” backup, in that it stores all unique data, and then on each backup figures out what it does not already know, and stores that.
There are no dependencies between versions (i.e. no dependency chains that can break), but they do share the stored bits so you save storage space.