A very slow, very expensive compiler - but it's still taking the source code (the training material and model architecture) and compiling that into a binary executable (the model).

Maybe it helps to think about this at a much smaller scale. There are plenty of interesting machine learning models which can be trained on a laptop in a few seconds (or a few minutes). That process feels very much like a compiler - takes less time to compile than a lot of large C++ projects.

Running on a GPU cluster for a month is the exact same process, just scaled up.

Huge projects like Microsoft Windows take hours to compile and that process often runs on expensive clusters, but it's still considered compilation.