Any stagger makes a "double stack" to me. A single stack is straight in a line like a 1911.Not a true double stack. Semi staggered was the closest name I could come up with while searching. It’s closer to being a single stack than a double. I think Sig is just pushing that “semi staggered” idea a little further. They seem to be fitting 10 rds in the same grip length as the shield’s 7rd flush mag without going to the full-size grip width of a double stack.
And for me the thinness of the Shield is it's selling point. I'd like to take a look at this Sig just for the trigger, except if it's thicker than my Shield I might as well go to a full double stack mag capacity. The points of carrying a compact to me are prioritized 1) thickness, 2) grip length, not much else matters (to me), barrel length isn't an issue at all unless it's too short.
If Sig gets the price point right I think they can give S&W and Glock some real competition.