I have believed it to be quite accurate myself.
So, two seconds per mile per pound over optimum weight. So if you are 10 pounds over optimum weight in a 10K (6.2 mile) you will be two-minutes slower. Seen this time and again.
Although extra tall people the math apparently is a little different, I think they become more acutely affected by extra weight.
I have observed this a lot. Also, in an interview, I asked Canadian physiologist and coach Trent Stellingwerff if it is accurate and he did an equation he had in metric and it converted very close.
Interestingly and very anecdotally I have observed (more as a photographer, but also as a friend or acquaintance of runners) people who compete for awards in age-groups. If 10 people get up (top-10 in their age-group, it is amazing, how the 10th is almost always heavier than the middle people and the lightest is most often 1, 2 and 3.