Once the transactions are confirmed in the blockchain, they are confirmed and no longer able to be double spent. Right?
No, this is incorrect. The block that contains the transaction could be orphaned. If block times are shorter, then it is more likely for a block to be orphaned, resulting in no difference in security.
Unintentional orphaned blocks would become more common, yes. But unintentionally orphaned blocks can't be used to double spend because you don't know when unintentionally orphaned blocks will occur, (and without resources you can't guarantee the new blocks contain your double spend transaction instead of the legitimate transaction).
But it would take resources to
intentionally orphan the last block in order to double spend a transaction with 1 confirmation. It takes no resources to double spend a transaction with zero confirmations. So, there is some security improvement when you compare 0 confirmations and a longer block time, to 1 confirmation with a short block time.