That line upsets me. They absolutely did not encourage teams to take advantage of existing services that they have previously written and pointing to one random post on the developer forum as evidence of this is terrible. My team and I read the official rules together several times before deciding that we could not in good faith incorporate our existing product (including using our own API to integrate features) into the mobile app we built for the hackathon. The line "built from scratch" in the rules seemed clear to us but apparently we were mistaken.