He would have had to mine initially to keep the network alive. In the early days people mined on their desktop CPUs, it wasn't common to mine consistently 24/7 so Satoshi likely mined for the first 6 months at a minimum. He was not the only miner but he would have been the miner of last resort. Some analysis has been done on the blockchain from the time and work done to tie some of the early addresses to known addresses of Satoshi (payments to more public early adopters). So whilst it is correct to say no one knows who he is we do know that there is a high likelihood that he mined and that he probably mined around 1m BTC. Those coins haven't moved but they are probably watched by thousands of people.