Call it what you want. Every human to a person experiences the ease of regaining previously built muscle. I'm a big science person but this "effect" is easily observable in every person I've ever encountered. I'm assuming you lift and have your own direct experience here over your lifetime.
If science doesn't have the perfect reasoning and causality down at this point then I have confidence it will advance itself eventually, but it doesn't negate the reality experienced by all which is nearly as observable as gravity itself (which also has no proven "why" but science observes and measures attributing its existence to mass differential but oddly no "whys" - it just is).