Not that many computers. Could be done in a relatively short time with a server farm or botnet.

Even now this coin is pretty small and the difficulty is much higher

I think it is more likely that the developer or a small team bumbled along for two years and then later decide to try to cash out the premine with a pump-and-dump instead of starting a new coin. That may be behind the split between bytecoin and cryptonote. All of this is speculation, but I'm trying to fit the facts as well as possible.