Um, not really any reason to doubt as there's nothing wrong with selling what you find works

Moreover, my experience in having formerly trained to compete at top international levels, studied exercise physiology and worked as a trainer, is very similar.

The really short oversimplified version is: more intensity, shorter training, and more rest — it is the balance of exercise and rest that is key. And world-class results are definitely possible with relatively brief workouts; in fact, it's the best way to do it.

The simplified concept is the muscles gain strength with stimulated rest. The training/exercise only provides the stimulus for the muscles to grow, the exercise does not actually grow the cells, it degrades or damages them. It is the repair process that strengthens the muscle. Too much exercise and too little rest (=repair+growth) just degrades the system; too much rest without exercise stimulus wastes potential growth time.

Some is good, but more is not necessarily better.

While there is no question that some exercise is almost always better than none, if you want peak results, intensity is the key. By intensity, we mean pushing the muscle to failure, so the end of each lifting set is not a predetermined number of reps, but the rep where you push as hard as possible and simply cannot complete the lift (after ~5-25 reps depending on focus on strength vs bulk, respectively). Do that one to three times for each muscle in the workout, then give it some days rest. A 45-min workout is sufficient to work the upper, mid, or lower body zone. Doing only one zone each day fits a max of six workouts per week, and monitoring vital signs (pulse/bp/temp) for overall stress will usually reduce that to around four weight workouts per week. This is what worked for me and the people I trained, and I'm not the least bit surprised to find it also worked for Dorian Yates (and no, I'm not selling anything related to exercise programs).