Gravity works both ways. I can assure you that you travel faster downhill than Craig with the same effort (wkg). I am sure the same applies when climbing.
Yes every one can go downhill “quite fast”, but still, the lightweight WILL most likely loose. In the real world maybe not, as out there your riding skills/technique can make up for low weight. In the virtual world with no corners, hairpins etc. to take into account, gravity wins.
When you look at @Goldwolfie‘s numbers, he is going like 2.5wkg max when leading the downhill pack, and the rest of us have to do like 3-3.5wkg just to hang onto his wheel.
I think it would be fun to have a couple of pure downhill / climbing races, where we switched weight, so the heaviest got the lightest’ weight.

It would be an interesting experiment.