I assume it just lined up with the conversation of Hal and Satoshi, but yeah, the funds had to have matured to be spent. IIRC, the original client would even wait 120 blocks before spending a coinbase output.

Murch

This is why the first bitcoin tx was sent at block height 170 between two people after the maturity of the reward to spend it? 

Nihsotas

bitcoin

Normal & Rare.

The only downside of overestimating the maturation period is that miners can spend their reward with a slightly longer delay. The downside of underestimating it is that you create an incentive to attempt long-range reorgs. 100 blocks to be on the safe side seems completely fine to me.