>I would expect it to require a proof that 1 - 2 is non-negative. That's kind of the raison d'etre of Lean isn't it?

The reason is to be able to write mathematical proofs, including proofs about your code, but not to attach proofs to every single function. This definition of subtraction does not prevent you from reasoning about it and requiring `a ≥ b` in the proofs/code for which this is really important.

>Requiring explicit proofs for every subtraction was presumably seen as too onerous.

Lean can deduce proofs implicitly as well. It’s just not a very reliable mechanism. That is, imagine your code breaking after an update, because Lean suddenly can’t deduce `a ≥ b` automatically for you anymore.

>Which is fine... BUT they then should have said "so we're going to define a more convenient operator which is LIKE subtraction but isn't actually standard subtraction, and therefore we won't use the standard subtraction notation for it".

What is a standard subtraction over natural numbers at all? As you know, under a standard addition natural numbers form a monoid but not a group.