Correct me if I'm wrong but afaik that's not accurate.
The 50coins in the genesis block can't be spent, but they exist nonetheless.
That is correct, but it is still a block that was mined and it wasn't created in the protocol by definition. Going by this definition, then Satoshi would have far more than 50BTCs, because he was the only person mining at that point in time.
The 50BTCs would still be a factor in the halving and also considered to be a part of the total supply.