In programming I knew I needed to sort a list or find a most efficient path because some practical problem I was trying to solve demanded that I do that. Frequently I had a basically crap but working independent solution before I learned the names "EWD" or "A*". I independently discovered that I needed virtual interfaces (before I knew them by that name, "I wish pointers to parent classes could call implementations in subclasses") and then discovered language facilities for polymorphism and OOP.
Without formal or at least guided instruction I would never think to move towards or discover "I wonder if there's a relationship that makes these double integrals of curls of vector fields easier to solve for".
Programming has a high coupling between necessity, experience, and theory. In mathematics that coupling is much, much, much looser. Self learners in programming regularly re-discover and re-implement, typically less efficiently, all sorts of fundamentals of CS. The equivalent in mathematics rarely happens post-algebra.
For example, take any good static analyzer that implements abstract interpretation. It generally works using Galois connections, which is just abstract algebra.
Dijkstra's algorithm or A* came pretty early in the history of CS. It would be fair to compare their difficulty to something similar in mathematics, say some basic results in Euclidean geometry.
If you're discussing pure CS, the thing you can write down in a book and for which a computer is a largely theoretical device, sure CS is mathematics.
If we're talking about the practical reality that CS majors in America today are trying to achieve, and their undergraduate programs are trying to prepare them for, that's becoming a working programmer and has very little relation to mathematics.
But so much of programming isn't. It doesn't require deep understanding, static analysers are advanced level things that are way beyond what many working programmers ever encounter.
I can't say I've ever seen "Real math" myself.
Second, even if you wanted to understand the proof of a theorem, doing it with a proof assistant is an atrocious way to go about it.
As a math teacher who disagrees with the premise of the GGP post ("This couldn’t be more wrong. … I’m sorry, but mathematics is orders of magnitude more intensive and difficult than most programming"), and thinks that any good programmer can learn mathematics—of course there are code wodgers out there who don't really understand their craft of programming, and so can't translate that knowledge to facilitate an understanding of mathematics—I think I also disagree with this. I've never tried it, but I can't imagine someone learning about Stokes's theorem in anything like this way. One of the many axes along which I imagine this failing are that the state of human readability in proof assistants is, well, let's say it's less well developed than the, cough, stellar state of the art in compiler error messages.
But, more importantly, you can, at least in principle, know every single step in a proof of Stokes's theorem without understanding in any real sense why it's true—and a proof assistant in particular will force you into the weeds of minutiae that absolutely do not help to build any intuitive picture—and, even if you manage in the process to piece together that understanding of why it's true, you will never thereby gain an understanding of why it's interesting (e.g., among other things, its connections to physics and the entrée it offers to differential geometry).
Input it into a proof assistant, and rely on the same sort of feedback "does the computer accept your proof, or get stuck". The hard job of formalizing stuff for this purpose has seen significant progress, e.g. by the Lean mathlib project.