Put simply: they are too fast for the beginner series and too inexperienced for the pro series.
Which means ultimately that they are dangerous, right? They drive too fast to be safe with the other slower beginners and they don't have the experience needed to be trusted to make good decisions with the people who drive in the faster circuit. It doesn't seem wrong, it just seems like a natural consequence of learning in a simulator.
I think that's the implication. The worry is that the claim isn't reliable. A driver who can control a vehicle at higher speeds than you will always appear "dangerous."
And similar claims were used to exclude women from the Indy 500 until years of women in F1 and NASCAR illustrated the prejudice was absurd.
It's admittedly a tough call though. On the one hand, the claim's being made by insiders with vested interests and skepticism towards outsiders. On the other hand, the cost of letting in dangerous drivers is far higher than the cost of prejudice, so there's a reason to err on the side of caution.