I don't know ... it doesn't seem like it is Alex's style to 'jump the gun' on these things, I gather he is happy enough with the current state of the game financially (to enable him to proceed development) so presumably he will want to wait for a bit before submitting it?
Just thinking about it, there is going to be a lot of 'noise' in the early stages of greenlight associated with lots of other developers eager to jump on this thing - and it could take a disproportionate amount of energy to give this a proper go - is this Steam-related energy better spent elsewhere?
I can see lots of success with Starfarer hitting Steam early, but likewise there could be a lot of pain (mainly associated with people being confused about the features that are clearly advertised but aren't in).
I don't speak for Alex, but for the long term success of the game, and for his own mental health, I would consider holding it back just now.