The HDMI Forum, responsible for the HDMI specification, continues to stonewall open source. Valve’s Steam Machine theoretically supports HDMI 2.1, but the mini-PC is software-limited to HDMI 2.0. As a result, more than 60 frames per second at 4K resolution are only possible with limitations.
Technically no, it has to specifically have Dual-Mode support (DP++). In practice most of them do, at least in the consumer space.
If it doesn’t then you need an active adapter.