That’s actually less true than it sounds. One of the primary functions of NaN is to be the result of 0/0, so there it means that there could be a value but we don’t know what it is because we didn’t take the limit properly. One of the primary functions of NULL is to say that a tuple satisfies a predicate except we don’t know what this one position is—it’s certainly is something out in the real world, we just don’t know what. These ideas are what motivates the comparison shenanigans both NaN and NULL are known for.
There’s certainly an argument to be made that the actual implementation of both of these ideas is half-baked in the respective standards, and that they are half-baked differently so we shouldn’t confuse them. But I don’t think it’s fair to say that they are just completely unrelated. If anything, it’s Python’s None that’s doesn’t belong.