I'm about 95% certain that CoE3 did not have to go through Greenlight.
And if the people at Valve were the hardcore gamers they'd have us believe, they'd have instantly recognised Dom3 and brought it straight in.
Perhaps COE3 didn't but they've also loosened up since Greenlight came about, too. I recall a couple indie developers being openly frustrated about being turned down by Steam a few years ago, with the reasoning that their titles' style "didn't fit into their portfolio" or some such. Which was pretty much First-Person Shooters and basic RTSes. Thankfully they loosened up a bit, or I don't think they would've even brought COE3 on-board.
It still sounds like a challenge to get indie turn-based strategy titles approved by Steam without going through Greenlight and that may well be the issue with getting the Dominions series in.
I'd still prefer to get it from Desura, since I can have both a client install
and a DRM-free installer version I can burn to a disk for safe-keeping.