Steam Frame needs 27-45W of power to run, and it can run off an additional battery
(media.piefed.social)
I think you're reading into it a little too hard. I don't suspect an optional mode based on what's written there. Those power specs come from the USB Power Delivery 2.0/3.x standards. There's 9 Volt USB PD that can supply 27 Watts at the 3 Amp ceiling, and the next step up is 15 Volt USB PD that can supply 45 Watts at 3 Amps. Since a 45 Watt charger is the one that's recommended, I don't expect heat issues there.
https://en.wikipedia.org/wiki/USB_hardware#USB_Power_Delivery