Is it really a backup? Ipfs has no 'upload' so if you clear your local storage, everything will be gone. I think it's unlikely that multiple users will generate the same content hash.
Right, it's a local, sharable copy. Actually Readability cuts off so much HTML, that it's quite possible that 2 users will generate the same hash even from very dynamic website.
Unfortunately it's unlikely to be a backup, since everything will only live on your local computer. You might get the content from some other user if, by chance, they generated the exact same bytes, but it's rather unlikely.
To back things up, you'll want to pin to an external server like Eternum.io.
If you export your bookmarks you could autoparse them and ipfs pin and ipfs links to a longer lived archival node running on you main host or offsite/cloud.