You are confidently incorrect.

SteamDeck supports HDMI-CEC as of 3.7 ~2 years ago, using the original dock or select third party docks.

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How to Enable CEC:

Press the STEAM button and go to Settings.

Navigate to the Display tab.

On the right side of the screen, find and toggle on Enable HDMI CEC Support.

Ensure Wake TV when device resumes from sleep is also enabled.