Transaction confirmation is an external concept which tells you how difficult it is (in terms of probability) to remove that transaction from the block that contains it due to reorganization of the blockchain.
Genesis block transaction is a special one as it is hardcoded in the source code and the entire Bitcoin blockchain is built on top of the Genesis block. Confirmation concept does not apply because you cannot remove this transaction from the Genesis block, as you will be building a completely different chain if you do so.
As a side note, the coinbase transaction is also not spendablecoinbase transaction is also not spendable.