It's a mixed bag. Some people don't use AI, for the usual reasons that people don't use AI. Others have used AI to write Lean programs. For example the 200K loc example mentioned in OP. But generally speaking, the quality isn't good enough yet. And when you've looked at 13 AI-generated examples and found them lacking on various axes compared to Mathlib, then you'll take a break before being motivated to look at example 14.