I don’t usually do well in competitions, but as the first respondent I think I might have a chance with this one.
Unfortunately it’s a little trickier than it first appears, as there is no standard system among different manufacturers for brush sizes, meaning this could be anything from 0 to 2. (A Raphael #0 is about the same size as a Winsor & Newton # 1 or a Da Vinci # 2).
I don’t have a Citadel brush to hand - they don’t last long enough, for one thing - but the black ferrule does suggest that brand, and if the screen shot is from an official Warhammer video I suppose it has to be, though this raises the question of how the point is so fine. I know some higher quality brushes, apparently rebranded Winsor & Newton ones at an increased price, were available several years ago, so perhaps this is one of those.
In conclusion, it’s complicated, but you could save a lot of money and anguish by buying from the three brands I mentioned first, with Raphael offering the best value for obvious reasons, as well as a high level of quality control.
Edit: Evidently I wasn’t the first respondent after all, thanks to my verbosity, but I hope my comments are of some value.