Active recovery "is a thing" because it works. In the context of cycling, going for a leisurely walk or a recovery ride gets more blood into your legs, or as my buddy put it "shakes the soreness out." It's nutty and unrealistic to think people have to do absolutely nothing until they've recovered from some exercise.
Also, muscle building and loss happen 24 hours a day. We're not talking about power lifting here, or even running; we're talking about riding bikes.
My doctor told me it's better be for me to stay active while I recover from a really bad case of tendinitis. I don't know how many of you have had it, in the foot and ankle no less, but it's harder to recover from than doing a long ride or a gym session. And the doc wants me doing, well, more than nothing.