No it's not a joke.
There's not much point putting a free licence on a code if it is designed to run only on non-free software. The user would have to modify the code to run it as free software, and would have to guess what the functions do in order to try to reproduce them with #SageMath or similar.
Many of us have no desire to endanger the security of our computers with non-free software, nor do we wish to depend on #FaithBasedSoftware such as Mathematica.