Is a payment returned through horizon api call: "Payments for Account" always "confirmed"?
In comparison to Ripple a transaction could be returned but not validated yet. Is Horizon filtering this out for us?
As of now (August 2018), yes. All transactions and operations (including
path_payments) you can see in Horizon are included in the ledger thus confirmed.
However there are plans to include failed transactions in Horizon DB too. This will likely change in a backward-compatible way but please track release notes for changes.