Yes, this is most likely not even used in this example. This was just an example of a place where the underlying implementation is dependent on the input size 
Also the underlying libraries we use like MKL and OpenBLAS will have their own thresholds. And the cuda libraries will have their own set of threshold (you can actually play with cudnn algorithm selection for conv by setting torch.backends.cudnn.benchmark and torch.backends.cudnn.deterministic).
I’m afraid it’s beyond my knowledge which thresholds are hit in this particular case.
If you really want to know, you can trace down the call stack to see exactly which function is used. And what are the conditions on that function. I would be interested to know the answer if you do that!