"Stable" is really an unfortunate label that I wish they would not use. They should just call it the main branch and the experimental branch period. The main branch is what Steam pushes out automatically to everyon who has their settings put to auto update. The experimental branch is what players must choose to opt into. All of this is early access and therefore not stable in the way that you are expecting it to be.
Did they rush to get A20 switched over to the main branch in time for the Steam Winter Sale? Yes and no. It was a goal to get the update on the main branch before they closed for the Holidays and, of course, to be in a good position for the sale. When the game switches to the main branch everyone who has the game installed is going to get notified that it updated. There are going to be a good chunk of people who have loved the game but who have moved on and haven't been following and when they get the notification that 7 Days just did an update they are going to check in and play it and they may have friends who they will now want to get into it and with it being on sale it is the perfect time. So, yes, having an update in time for the sale is great for increasing the reach of the game. Nobody is denying that this was their goal.