Now, of course, if you are truly paranoid, you'll want to review their code first. I don't get why I can't simply mount a volume with encription and write there. Using code that is already on my machine (on the kernel, nonetheless) would make it a much simpler decision.
You can (and you could even use tarsnap to back up the encrypted filesystem image if you want), but writing your data to an encrypted filesystem tends to expand the amount of data changing -- in the extreme case, if you create a copy of a file you'll write that many blocks of new encrypted data which needs to be backed up, whereas tarsnap would just say "hey, I recognize all these blocks, it's those ones I backed up earlier" -- so Tarsnap's encrypted backups of a filesystem tend to be many times more efficient than backups of an encrypted filesystem.
If you want to mount an encrypted filesystem stored on S3, you might want to try ObjectiveFS. https://objectivefs.com