Stress and lack of sleep will kill your gainzzz quicker than anything. Easier to manage at my age, and I notice it quicker now days when I'm off the beam. No more magic wand of youth to compensate.
I listened to the interview mentioned in the video. If I understood correctly, the author pretty much concluded that everything else was largely rounding error or placebo.
As usual, just sticking to the basics will give the desired returns. We have an entire fitness industry built around reinventing the wheel.