Can I respectfully disagree on that one? I find going downhill, I can easily use the more technical lines on the trail the uphill rider can’t, since I’ve got the speed and momentum. The uphill rider has much more limited options. I would rather see them just stay in the groove, albeit as far to the right as possible. But as in recreational sailing, a right of way is not carte blanche to be a jerk. Rather, it means maintaining your line, letting the downhill rider make the pass. If done right, rarely does anyone need to stop, even on seemingly quite narrow trails.
Maybe we are interpreting it differently. My interpretation is that to yield the right of way means to actually stop or at least reduce speed to a crawl until the other rider has passed.
Otherwise, I agree with you that when both riders continue rolling on a narrow trail, the downhill rider often has more directional options, due to higher momentum. But I guess I viewed the situation you presented as one in which nobody really yielded; both riders recognized that they could keep moving and each picked a line that both allowed them to proceed and allowed the opposing rider to proceed as well.
This tends to be the normal situation in most passes of riders going opposite directions, and I guess I saw your scenario as an instance where neither rider felt the need to apply the IMBA rule. But I have certainly run into instances where the uphill rider faulted me for continuing to roll past rather than stopping completely, even though I had picked a downhill line that didn’t impede him.
When doing this, you of course have to make allowances for the apparent skill or lack thereof on the part of the opposing rider; you don’t need to allow as much room for a skilled rider as for one who is obviously less confident.
Of course, on multi-use trails where horses, dogs and pedestrians can be present, the bike rider, whether going up or down, should be sure to yield or pass in such a way as not to surprise them.