[kang]

So manual then?

To reach the ethereum genesis block programatically, starting from an erc20 token I give you, you need that "need the contract code" part which is centralised.

[decentralised]

There is a difference between tokens and the currency Ether so I'm not sure what you mean with the genesis block in regards to ERC20 tokens here.

The contract that holds the ERC20 is hosted in every single node of the blockchain so it is not centralised at all.