I am confident you would observe the opposite result if testing someone who did not grow up using d6s with pips. It’s not the pips or numbers, it’s what your brain is trained for.
Though it would be worth testing, since one counter-hypothesis would be that, as the pips are a literal representation of quantity while numbers are purely symbolic, perhaps brains might have an edge in intuiting the amount at a glance. At least with a limited number of dice.
Okay, now I’m thinking there could be some interesting findings here. I’d like to see an experiment done. For example, a result where pips had the edge with fewer dice and numbers with more dice could suggest some things about the interaction between intuitive counting and symbolic logic. Lotta variables, though.