Yes but it goes deeper than just corpus, because not all programming languages are created equal. More statically enforced language perform better with AI because the tooling catches more bugs earlier. Highly dynamic languages like JS are very poor rewrite targets, despite being popular, because you essentially have to live in the debugger and employ maximum TDD to make it work reliably. And AI doesn't do that.

Consider just having a typechecker. That alone eliminates a whole class of bugs and unit tests, which means your remaining unit tests check more logic. I've seen JS projects with high test coverage that cover essentially zero logic, only type checking class things. And nobody even knows it, really. But when your tests are just "okay pass an empty array. Now an empty string. This should fail, great. Now pass an array with the wrong elements..." you're not testing anything. You're probing the type system, which many languages do automatically and perfectly.