His logic is a bit off. In a truly efficient market (big if), if taxes were raised so significantly as to have a major impact on the rewards, then you would get a corresponding reduction in risk. I.E. Instead of having a 1 in 10 chance of a startup making it (as he claims in the post) you would probably end up with a 1 in 8 chance (or something similar) as the smaller rewards would, in theory, reduce the number of competitors. You would still end up with the same number of winners however.
In reality, there are a number of intangible benefits to starting a business that will overpower any reduction in startups due to higher taxes. Autonomy, excitement, and having a large impact all contribute in the decision to take the risk. Additionally, starting a business is still one of the only ways to strike it rich (unless you get lucky in the athlete/musician/actor department). So for most people with sizable ambitions, a reduction in rewards due to higher taxes will not have any meaningful impact on their decision.
In reality, there are a number of intangible benefits to starting a business that will overpower any reduction in startups due to higher taxes. Autonomy, excitement, and having a large impact all contribute in the decision to take the risk. Additionally, starting a business is still one of the only ways to strike it rich (unless you get lucky in the athlete/musician/actor department). So for most people with sizable ambitions, a reduction in rewards due to higher taxes will not have any meaningful impact on their decision.