Interesting.
I was only reading the other day about making barley wine and other high gravity beers.....the article explained that the sparging process is often incomplete and some sugars are sacrificed to maintain a low volume of wort. It then said you should sparge again and collect the low sugar wort to brew 'shandy'.
I would guess that Brewdog have quite a bit of shandy type incomplete spargings.....fair play with the marketing wright up to.