Lean's maths library development is essentially completely focussed on classical mathematics, which is why it has been so successful in drawing "working mathematicians" in. Such people do not care at all about the issues involving quotients, indeed quotients in Lean work just fine for them, and they don't care about constructibility either, because this is the prevailing culture in maths departments (although it is not, I am well aware, the prevailing culture in computer science departments). If you are interested in constructivism I would not recommend Lean. Many of the developments in mathlib are classical. That's why it's moving so quickly -- classical mathematics is much easier.