There are a few ways to make a creamy chile sauce.
1 part of sour cream to 1/4-1/2 part of mayo is a good start. I checked the site and they describe it as a "creamy red chile sauce", which is pretty vague. I'd have to try it myself to know which route they took.
If you add pureed chipotle chiles in adobo into that mix, that's one way. If you don't want the smokey flavor, but still want it to be a red sauce, adding pureed roasted red jalapenos and/or roasted habaneros would work. Then at that point you can add any sort of powedered chile for more spice.
If you don're care about the color, adding avocado and roasted jalapenos is pretty good too. The avocado adds another level of creaminess and another depth of flavor.
There are a bunch of different kinds of peppers you can use too, however some are hard to find if you're not near a large supermarket or specialty store.