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From the front page: "Optional P2P backup, so even if your Own-Mailbox is offline temporarily you don't lose any email from your self-hosted addresses as long as you maintain 70% up ratio." and "16GB storage, possibility to extend memory or do backup via USB drive/HDD."

So really, it has both off and on site backup.

That sounds more like an MTA availability backup, not a data backup solution...

I'm also wondering how the P2P backup works exactly?

As an example, if it comes with 16GB storage and my mailbox is 10GB, does that mean other people's boxes would contain the full 10GB dump or just pieces/blocks a la bittorrent?

If you use an IMAP client, you have a local backup. Alternatively, even if you exclusively use Webmail, you can still maintain a remote backup (using imapsync or similar). There's also the "optional P2P backup".

You really should be mirroring on your local computer which you back up locally as well