Which public key to you hash as the secret? obviously not one that has been made public I assume?
Right!
The secret for SwapBill pay on reveal secret transactions is basically just a binary string which happens to have the same length as a public key, and which can then be hashed with exactly the same code as is used for generating a bitcoin public key hash from bitcoin public key.
But this secret data is not actually used as a public key.
The secret data is managed separately from other public key data in SwapBill wallets, and is not used as a payment target, or publicly revealed in any way, except from in explicit 'reveal secret' transactions.