No, that's just the only way to get a game put forward for Greenlight. There are other conditions which attract Valve's attention such as winning awards, or putting forward a very convincing case at E3, GDC, etc.

A famous case in point was Kerbal Space Program, which had several threads devoted to it here and in the "requests" sub forum, and also had as many people posting equally futile threads to KSP's own forum about Greenlighting (it got so bad that people could be kicked from Kerbal's forum for even mentioning the word "greenlight") The devs maintained that when they were ready they'd be on Steam without a vote needed, but for months idiots persisted to ignore the devs, the moderators, the fora, and everybody else, while posting identical threads here and there.