I might be talking out of my ass because I only recently have been reading/studying your books, but I can give you my 2 cents on why he thinks this from my limited perspective.
The train of thought must have arrived from the use "active recovery" for endurance athletes. In my experience, endurance athletes only do active recovery rides to recover from intense rides to "flush out the legs" and to enjoy themselves. As you mentioned, the metabolic system is completely different. Heavy weight lifting will not produce the sacroplasmic waste as say running for 30 minutes. The prevailing logic from this concludes "active recovery" is not necessary for heavy weight lifting. Furthermore, it is slightly detrimental if you're trying to gain weight.
Correct me if I'm wrong.
P.S.
I just started taking classes to get my degree in physiology, but from the impression I have now the only benefits is the anatomy, nutrition and the basic purpose for any given subject. None of it really helps for training. I don’t know what SBS is sometimes and what’s not in the academic world.