Not necessarily. Theorem provers provide goals that can serve the same function as "debug text." Instead of interpreting the natural language chosen by the dev who wrote the compiler, these goals provide concrete, type-accurate statements that indicate the progress of an ongoing proof.

I'm referring to what the authors actually claim they did in the paper. They operated on XLA-generated textual IR.

Cf. the second paragraph of 3.3.4 of https://storage.googleapis.com/deepmind-media/DeepMind.com/B...