It doesn't explain why 50% of the coins are not held by Dev.
It doesn't explain why 250% of the coins are not held by Dev either

Premine = the dev creates a coin where a number of coins (from 0.0001% to 100% of the coins) goes straight to a certain address and is then used/distributed etc.
Instamine = fast mining (where many are mining). Example = PoW/PoS coins which the entire monetary based is instamined within a week. Now, just because something is mined fast doesn't mean the dev has 100% hashpower. Go to the Darkcoin thread and look at the miners and their WTS 10.000 DRK for 0.25 BTC.
Darkcoin is currently at 80,000 blocks, but theres only 4.3m DRK out there. Five months later, the 1.75m generated during those first 4200 blocks still represents 40% of all DRK in existence.