Yup, exactly.
It used AuthentiCode executable signing to bind VeriSign's timestamps to the documents. Basically, this allows for an unlimited use of VeriSign's mature and robust timestamping infrastructure for a cost of a single signing certificate ($150/year or thereabouts). Not to pat myself on the back, but I think it was nothing short of ingenious :)
That being said, I really like how the service worked. If I understand it correctly, I would download the signed work (signed .exe with content included), which also means that I would have proof after the service got shut down.