You are thinking today and not the situation when the pizza was purchased. That is kinda out of logic IMHO. Besides, just like what @Becassine stated, we never knew the person, his character, and the amount of BTC he is holding.
Bitcoin value, price and Bitcoin blockchain difficulty at time of Bitcoin pizza trade were very different and considerable lower than what they are in 2025, therefore people thought differently in 2009 than in 2025 is very understandable. We even don't need to go too far to the past such as 2009 year and with nearer years like in one or two previous market cycles in 2017 or 2021 year, I believe that many people did not think Bitcoin will be exchanged around $100k so they sold bitcoins at $1k, $2k or $10k. I think most of them regret now but if they sold their bitcoin with good profit, I think they did not do anything wrong.
Is not, it is an important milestone in Bitcoin history, and you never knew whether it is just a fraction of his BTC, besides, BTC was given away during those times too, like faucets where every drop gives 5 BTC to an individual who claims. Are you saying that person is a fool too?
If he reserved even small part of his whole bitcoins and managed to hold it till the last market cycle or this market cycle, I believe that he has no reason to regret. Assuming he had 10,000
BTC in the past, and lost most of it but managed to keep like 10 or 100
BTC till 2025, I do think he would still feel happily enough now.