The output of the training engine, I.E. the model itself, isn't source code at all though. The best approximation would be considering it obfuscated code, and even then it's a stretch since it is more similar to compressed data.
It sounds like Meta doesn't share source for the training logic. That would be necessary for it to really be open source, you need to be able to recreate and modify the codebase but that has nothing to do with the training data or the trained model.
