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.

How is that relevant here? You should have named Android [0] because that is only platform that has more users than Windows.

[0] But that one is still Linux.

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".

missing z/OS as well...