They must be doing something else with numpy as Python has a generic API, that allows direct access to data of numpy array-like structures: Buffer Protocol https://docs.python.org/3/c-api/buffer.html
Buffer protocol means PyTorch can have a torch.tensor method, that can create a tensor of anything implementing this protocol (which NumPy arrays do), and likewise numpy.array can be constructed out of PyTorch array because PyTorch Tensor implements this protocol. E.g. they don't need to know about each other (so no dependency), only about the protocol.
