Even at Wolfram Research, Theodore Gray is credited with inventing/developting THEIR initial interpretation of the notebook interface. His Wikipedia page [1] makes that clear.

1: https://en.wikipedia.org/wiki/Theodore_Gray

I'd guess MathCAD was quite a bit more popular than Mathematica in the nineties, given Mathematica was something like ten times as expensive.

I certainly won't blame anyone for saying that, given that it's repeated so often that it's turning into common knowledge.