The fact of the matter is that the only real differentiator between Zumodrive and Tarsnap is the integrity of the code that implements the service. Nobody is going to break TLS in order to get Zumodrive data.
Cryptography has exactly one implication for attackers: it is an opportunity to cause unexpected things to happen at the target. This is only possible when you do something unusual with cryptography. From that perspective, Tarsnap worries me more than Zumodrive: there is nothing that is going to go wrong with their SSL implementation that I'll be able to take advantage of, and I can't say that about your contraption.
(Here you will inevitably point to a string of OpenSSL security advisories to attempt to refute this point. You won't be convincing, for two reasons. First, it's been a long time since someone found an OpenSSL flaw that would move the dials for an attacker in a basic one-way-authenticated SSL session. Second, no security practitioner would trust your code more than OpenSSL, which, while terrible, has had something like 15 years of audit exposure.)
From what I can see, nobody professional has stated that they've looked at Zumodrive or Tarsnap. I'm pulling for you, Colin, but you haven't earned the right to impugn other people's security.
You should rewrite your article. Instead of trying to tear down a competitor, you should write an article about how Tarsnap is different from other backup solutions. You should write about how your threat model is different, and how you believe you're the only solution that has been architected to encrypt backup data in such a way that even the hosting providers can't access it.
You blew a bunch of credibility by trying to make these points while sniping at a competitor. Instead of thinking about how great Tarsnap is, I'm left questioning the objectivity of every line you wrote. What a waste! There's a reason every business book in the universe says not to disparage competitors. Go read one!