Last time we tried to figure that out we had some discussions internally and found the available information at least unclear - and indeed were not looking at this web page but at the concrete data sheets of the CPU we us, i.e. the 10710U. The different CPUs indeed support different levels of Display Port and from what we knew from Display Port specs from the Librem5 development it was at least unclear to us if it would really fully be supported, especially being routed through the type-C.
Now we know 
Cheers
nicole