Guinness creates their Guinness Flavor Extract (GFE) that is apparently just roast barley steeped in their very low alkalinity water. That should produce a liquor that has a pH nearing 4.5. That's low for a typical wort, but not 'sour'. I do not know if they actually sour the GFE. They combine that GFE with the regular wort composed of raw and malted pale barleys. That combination of substantial roast content, low pH, and regular wort help produce that sharp, roast flavor. That flavor and its moderation by the huge body imparted by the raw barley beta-glucans and raw barley flavor are what create that smooth Guinness flavor, IMHO.
I feel that you can get reasonably close to the Guinness dry stout result by adding the roast barley as a late mashing addition. However, as many pro-brewers and multi-Ninkasi winner Gordon Strong can attest, having the ability to Blend the beer components, gives you much more opportunity to fine tune the final product to taste and uniformity. I think that is why Guinness uses the GFE. Another reason is that their other breweries around the world may not have had access to low alkalinity brewing water and they could easily export the GFE to the other breweries for blending on site.
Guinness dry stout brewing is an interesting study, but I have to admit that I prefer other stouts and porters with smoother roast character.