The jargon is a bit confusing sometimes. In Lean, "refl" does a whole lot more than prove x=x. It is of course available if you want to prove x=x, but the real power of "refl" is that it also proves x=y where x and y are definitionally equal. Or at least that's the idea; it turns out that lean's definitional equality relation is not decidable so sometimes it will fail to work even when x and y are defeq, and this is the theoretically distasteful aspect that came up on the linked Coq issue. In practice, the theoretical undecidability issue never happens, however there is a related problem where depending on what is unfolded a proof by "refl" can take seconds to minutes, and if alternative external proof-checkers don't follow exactly the same unfolding heuristics it can turn a seconds long proof into a thousand-year goose chase. By comparison, methods like "simp" have much more controllable performance because they actually produce a proof term, so they tend to be preferable.