If I had to hazard a guess at why the gut reaction is to call it a joke, it would be that (1) meal replacement shakes have been around for at least two decades

If I had to hazard a guess at why the gut reaction is to call it a joke, it would be that (1) meal replacement shakes have been around for at least two decades and (2) it's not a weight loss suggestion (although he mentions it may be useful for that).