You will see that at that time, early 2010, Bitcoins were pretty much worthless. It was simple to get them, you could just click on a computer and after a few days or weeks you would end up with thousands of Bitcoins. The block reward at the time was 50 Bitcoins, the difficulty was very low, and there were very few miners.

Laszlo was an early Bitcoin miner, he even published the first GPU miner. So basically he ended up with free pizzas, he traded computer generated coins for real pizzas. At the time it was a great deal, you couldn't get anything for Bitcoins, so getting pizzas was a great thing.

If you read the comments, you'll realize that it was actually hard to find someone to provide the pizzas:

Happy Bitcoin Pizza day!