Yes, that's correct. You have a gamepad, so the syntax would include the btn part, as you've already noticed - I realized after posting the example that the configuration part is covering the default keyboard mapping in RetroArch.

Using the RGUI you can also check what values are assigned to each button, just to check if your mapping are considered. Note that you cannot use 1 single gamepad for 2 players - as you've posted, RetroArch doesn't support that (it does support to use 1 keyboard for 2 players though).