Maybe it's a myth, but it sure sounds plausible. The software that checks the "Windows 9" substring doesn't even have to exist for this to be reason they chose to skip to version 10 — they just had to be concerned that it might exist.

Sure, maybe there's no C function that returns the string, but there's a ver command. It would be trivial to shell out to the command. https://en.wikipedia.org/wiki/Ver_(command)

This doesn't prove anything, but there are a TON of examples of code that checks for the substring. It's not hard to imagine that code written circa 2000 would not be future proof. https://sourcegraph.com/search?q=context:global+%22%5C%22windows+9%5C%22%22&patternType=keyword&sm=0