We used AIVAT to reduce variance, which reduces the number of samples we need by roughly a factor of 10: https://poker.cs.ualberta.ca/publications/aaai18-burch-aivat...
Furthermore, Chris Ferguson, scumbag aside, is absolutely still a very good player by today's standards, and one way higher than the mean participant in a research experiment.
10,000 hands is an effective enough sample at a certain win rate and analysis of variance of play; the n-value alone is not enough to tell you if it was enough hands.
FWIW, they did some variance reduction techniques that dramatically reduce the number of hands needed to be confident in your results, so the number of hands may be bigger than you think. e.g. the results of 10k HU hands have much higher variance than the results of 10k HU hands where everyone just collects their EV once they're all in.
It is an interesting point that these are pros but their specialities are either tournament or heads up. The current 6 max pros are LLinusLove, Otb_RedBaron, TrueTeller.
Is 10,000 hands really considered a good enough sample? Most people consider 100k hands w/ a 4bb winrate to be an acceptable other math aside. However, as your opponent and yourself play with equal skill, variance increases to the point where regs refuse to sit each other.