These lessons are aimed at getting someone up and running with a solid knowledge of constructing a figure in three dimensions as quickly as possible. You can quite easily stack anatomy on top of this foundation and draw in more complicated styles than "1940s cartoon character"; it's useful to learn construction with those kinds of characters because they're basically little but their construction.
I do not by any means discount the value of anatomy lessons and text, life drawing, sitting out in public doing quick observational sketches, frame-by-frame analysis of video, going to the zoo to draw, or any of the other myriad activities that make up part of a serious art education. I did that, and so did everyone else I knew during my time in animation, no matter how stylized most of their final work was.
But I think these lessons are a great hand up over the huge gap between "I can copy what I see onto paper OK" and "I can draw any damn thing I please in any pose I like, with no reference beyond a few design drawings".
(tangential edit: I'm surprised to find another cartoonist here on HN and curious to see your work, but your website doesn't mention that at all, and Google turns up nothing. Any links? My website[1] is full of my stuff.)