Not universally true. I would assume that an average Series D startup has way less risk of evaporating to zero than a seed stage startup. With that in mind, the risk hedging might actually work in favor of "six bets rather than 30". But you will need actual numbers to do that risk assessment, and I assume VCs do that.
Let's say you invest $300mil into 30 seed bets, with each bet having a 10% chance of returning a 10x, 15% returning a 3x, and 85% of going to zero. But when you invest that same amount into 6 series D bets, each bet might have a 50% chance of returning 2x, 30% chance of going to zero, and 20% chance of going 3-4x. If you do the math to calculate the average expected payout using these numbers, you will get an expected average payout higher for the latter scenario. And assuming each bet is completely independent from another, it seems like a pretty solid hedge.
Numbers are obviously made-up for illustrative purposes and are not the source of truth, but it shows a pretty good hypothetical situation when doing 6 bets is safer than 30 bets (given you have the same amount of money to spend on those bets). But, I think, it is fairly commonly agreed on that a Series D startup is way less likely to go to zero than a seed stage one, thus making a singular bet on a Series D startup much safer (but also less profitable in case of a success). Given many enough of those bets, the risk becomes pretty manageable and, imo, less risky than seed stage investing. I am not trying to say that what I am describing is the case, I am trying to say that it is a realistically possible case.
Saying this as someone with no experience with that model whatsoever, so anyone is welcome to correct me if there is something glaringly wrong or missing in my assessment of this.