I'm familiar with manga and anime art, pro or fan. This is not the level of anatomy we were talking about. Not even close. Classical anatomy drawing is this kind of thing:

https://www.google.com/search?q=classical+anatomy+drawing+sh...

https://www.google.com/search?q=classical+drawing+florence+a...

This does have direct application in american superhero comics. There the artist draws detailed muscle modelling and attempts to place tendons and shape of underlying bones all in the right place. Even when the character is really not human :-) I looked for one but wasn't able to quickly pull a video showing a critique arguing that some muscle is not quite located and anchored correctly.

I rarely see that concern in manga or anime. Very little in the video you linked would show such a concern. The usual manga artist does proportions, yes, often altered to serve their intent - but in my mind rarely goes to that intention of physical correctness.

Let's try and pull some manga muscle ... this is NOT classical drawing or derived from that:

https://www.google.com/search?q=manga+sports+muscle&tbm=isch