PaterAlf: Yep, unfortunately the developers went for steam-first. I'm a backer and even we didn't get the new mode yet (think we will get it around the same time as GOG).
I don't really need it as I don't think the game is very hard (except for the last boss), but it would've been nice to treat all customers more or less the same (wouldn't mind some days, but several weeks or a month is definitely is too much).
According to Twitter, they've said it's for budget reasons, etc. They also compared creating patches for Steam, GOG, etc. as being like porting to Linux and Mac, which is rather worrying.
In reality, I think it's because Steam dangled a carrot via trading cards and they wanted to push that patch first to hit the biggest market and get some money via the trading card system.