Announcement on the official Unreal Engine website.
And yes, the announcement does target Steam specifically, as you can see in this graph on the page:
Essentially, they're trying to attract developers by offering a bigger cut of the sales. Apparently, there will be more details at The Game Awards.
I dunno. On one hand, competition is good and a strength of the PC platform. On the other hand, this is an already very crowded market - I'm personally already juggling between 5 different game clients plus Windows Store. I expect a hard grind for even any kind of foothold.