Basically, although code and proofs are in a sense isomorphic, I think everything and proofs are isomorpic in the same sense. So claiming that proofs should be structured to the same standards as code because code is isomorphic to proof is a bit of a non-sequitur. Why not show the layout of my rock garden is isomorphic to code and hence proving something (my rock garden can be Turing complete) and then claim my rock garden should be structured like a program?
Basically, the idea that the structure mathematicians favour for proofs have fallen behind programmers laying out of code is not well substantiated and the idea that 'proofs are isomorphic to code' is not useful because it is far too broad.
All of the relationships between these are the same too (e.g. a program `p` has type `T`, iff the proof `p` proves the proposition `T`; the proof `w` proves that the existence of `x` implies the proposition `y AND z` iff the program `w` is a function which generates a tuple of `(y, z)` when given an argument implementing the interface `x`; and so on).
Mathematicians and programmers do care about different things. Mathematicians are more concerned with informally proving previously unknown results; which corresponds to something like learning that a type has a value by sketching some pseudocode. Programmers care deeply about how many steps their proofs (programs) take to simplify; how easy it is to modify a proof to prove a slightly different proposition, etc. Yet there are so many good ideas on each side that are immediately applicable (e.g. dependently typed languages take rigorous logical systems and view them as programming languages; proof engineering takes software engineering ideas and applies them to building and maintaining proofs).
I'm struggling to think of an activity involving rock gardens that corresponds precisely to, say, universal quantification.
Of course, that still leaves the same problem of explicit proofs being tedious, i.e. being able to prove pythagoras's theorem "with a diagram" does not mean being able to prove it "within an explicit, formalised diagram language".
I don't think I agree with this statement. As a matter of fact, I've always had a deep unease with so-called "geometric proofs", where a succession of visual transformations of a diagram are used to prove a theorem.
You can certain explain the intuition behind the proof of Pythagoras theorem with a diagram, and there's huge pedagogic value in doing that.
But to me it isn't a formal proof until it has been codified in a language that a computer can walk from start to end making sure each step is valid.