Not an ordinary payment to a pizzeria
Hanyecz posted his offer on BitcoinTalk on 18 May. Four days later another user paid for the pizzas conventionally and received the bitcoin. The restaurant did not need to accept Bitcoin directly.
This was a mediated exchange between two people with different wants. Markets begin when counterparties voluntarily agree on a ratio of exchange.
A trade helps discover a price
The pizza created an intuitive price anchor. Bitcoin no longer looked only like a reward inside software. Very low liquidity meant finding a counterparty took days, so one deal was not a precise global price.
Price discovery is the process through which offers, demand and completed trades form a market price. Early isolated trades are noisy, but they provide information for the next trade.
The hindsight trap
Multiplying 10,000 BTC by a later price makes the purchase look absurd. Hindsight bias makes a known outcome seem obvious in advance. In 2010 nobody knew whether the network would survive, gain users or retain any value.
Spending also helped create the use that could make the network more valuable. A historical decision should be judged using the information, risk and purpose available then—not only a price observed years later.