This analysis doesn't actually prove the title, as it only considers founders that have gone to OpenAI or Anthropic, however even if it did it isn't particularly suprising.

Sam Altman was the president of YC, so it's not particularly suprising that he has hired lots of talent from YC - these are people that are pre-qualified because he has seen their work before outside of a job interview (better the devil you know than the devil you don't).

This happens everywhere - person leaves company A to company B and then poaches the best talent from company A. Obviously in this case the talent are founders rather than staff members, but still.