Funds came from the first 1000 blocks which were instamined. Difficulty only kicked in at block ~1000, dev probably mined about 2.5M SIGT however it was spread across 1000 wallet addresses and has been transferred around into several wallets holding <100k each. Have a look in the block explorer at any block 1-1000 and then follow where that miner's funds went and you'll find a block where ~10 wallet addresses transfer their funds into another wallet (which can only be done using the same private key for all input addresses).