I think it’s both fair to say Linux and Mac make something cross platform, but it’s also very reasonable to expect Windows as well given its massive install base size.
But defining it by total users isn't very useful either - in this case, "total users who regularly use a terminal" would be a far more relevant factor. I think the only reasonable, useful, general definition of "cross-platform" is "runs on at least 2 of the 3 main desktop platforms".