DP 1.2a goes up to 21.60 Gbit/s, and 4k 60hz (which does work fine over 1.2a) needs 12.54 Gbit/s. I'm not sure how the 5Gbit/s and 10Gbit/s USB-C cables fit into this. It's obviously not just based on the cable's "rated" bandwidth, as if it was neither the 5Gbit/s nor 10Gbit/s connection would work at all for display. But it does, so it must not be, so what is it?

Is it 5Gbit/s per twisted pair? That would make sense as displayport uses 4 pairs so 4 * 5 comfortably gets you the 12.54 with plenty left over for regular data. But then it would matter for the 10gbit/s cables if it's talking about superspeed 1x2 (2x5gbit/s) or 2x1 (1x10gbit/s).