You also need a powered adapter for HDMI 2.1 in this case. The passive adapters work, because the DP output on the computer usually supports switching to HDMI output. But for that to work, the driver must support it, so it has the same problem as the HDMI port (which supports 2.1 on a hardware level, but not with AMDs open source drivers)

If you want 120Hz, only active adapters will work, which cost around 40$. If you don’t, the included HDMI 2.0 will be enough

If you don’t need 120Hz, you don’t need HDMI 2.1. You can get 4k @ 60Hz with the HDMI 2.0 that the Steam Machine has, so you can use just any TV.

Passive DP-to-HDMI adapters only work if the device on the Displayport end supports Dual-Mode, i.e. using the Displayport to send an HDMI signal. They often do, but it would require the same driver support for HDMI 2.1. So this would require an active adapter.