Here is an answer from @jack of Radxa Team: “you can power from the type C port with dummy 12V, up to 20V”.
Another quote from here: “Some dummy 9V or 12V adapter will work. … For QC, currently we select 9V because it’s the most common. For PD, we request 20V first, if not then 15V, then 12V, then 9V.”
I wonder who are wrong - you or @jack?