> In my mind STEM topics exist on a scale that has math at one end, then there's a slight pause/discontinuity (as in point 2 in the underpants gnome business plan) before it continues with physics, chemistry, biology and onwards to god knows what.
Whether or not it is a joke probably depends on how you label the axis along which you distribute these topics. :)
Observe that as you move from one side to the other (math to biology and onwards), you are essentially dragging along a good portion of the stuff as prerequisites you have to know a bit about.
By the time you reach something like immunobiology you can pretty much build a house out of the textbooks and go live in it. :)
The joke as I heard it in the 80's -- and I have no reason to think it was new then -- went something like: Physics is just applied math, chemistry is applied physics, etc...
I've always liked the view that Physics is just experimental discovery of the axioms "chosen" for our universe. Physics without experimentation just being a specific subset of math.
https://xkcd.com/435/