Hello bolo,
You are correct, in that the AMD card can output 5.1 surround sound via pass through and allowing an external device to do the decoding. The problem there, is the software controlling the sound device must support pass through as that isn't an option on the standard windows desktop.
You can try setting your speaker configuration to 5.1 in Windows 10. I have a PC connected directly to a Marantz receiver with HDMI and I do get all five speakers to test appropriately when I set the speaker configuration to 5.1.