It’s not entirely clear to me exactly what was meant by the grandparent post by converting to PyTorch or Tensorflow.
There are two discrete concepts: training and inference. You can think of training a bit like the source code and inference a bit like running the compiled binary. The different frameworks have their own serialization formats for their model weights.
If the goal is to do inference using CoreML weights trained in CreateML on non-Apple platforms (eg on a server or android device), converting them to ONNX is a way to do that.
You probably won’t be able to pick up training on another framework though.