They are not quite the same thing, DejaDup essentially copies files from one place to another, presumably an external drive or something similar. Then, to restore the files you would copy them back. It's a bit more involved since it uses its own internal binary format but that's the gist of it. It's better suited for personal files that change very often.
Timeshift works by taking incremental snapshots of the filesystem in order to reconstruct it step by step. The idea is that if you install something and it doesn't work, or it breaks something else, you can rollback to a previous snapshot. Since snapshots are incremental, it would be a mistake to use it for personal files that change very often, that would leave you with massive files that a lot of space and are slow to work with later on.
So, Timeshift and DejaDup actually complement each other nicely.
I saw in another post you said:
Timeshift works much better when using BTRFS, as opposed to fallback to rsync. That explains why it's much faster. Otherwise, I wouldn't take snapshots that often either.