Yes and no - I think you're thinking of very early stage startups that are still looking for PMF (which indeed is also a big reason why many are in the Trough of Sorrow). But the Trough of Sorrow can come at many points during a company's lifetime, not only in the very beginning.
In my experience, many companies that are still startups hit a growth stall later on, let's say 3 years after launch, maybe even 10 years after launch - at this point it can be very valuable to look at positioning.
I agree about evaluate positioning 3 yrs in, assuming you’ve hit PMF.
But as long as you have PMF in a big market, there is no trough of sorrow. These two things are mutually exclusive, and if you have the sorrow, you do not have PMF.
I say that as someone who has lived both at the extremes.
Fwiw, PMF is not an end state. It’s like riding a bronco. When you have it, you may feel fear, but never sorrow.
I think it depends on how you define trough of sorrow, there are many versions this phenomenon can manifest. In a company I consulted, they had PMF in one segment but the segment was to small to generate quite serious growth - hence they hit a trough / growth stall. While going after a more lucrative segment, competitors started to catch up - diminishing profit from the original segment as well.
Turned out positioning was the problem - they still had PMF in both segments, the issue was that they were not able to communicate what they offered properly, leading to leads choosing competitors instead. Changing their comms put them back on track.
In my experience, many companies that are still startups hit a growth stall later on, let's say 3 years after launch, maybe even 10 years after launch - at this point it can be very valuable to look at positioning.