~20 years ago this was true (for the US at least, and they had run into antitrust problems as a result) but nowadays it's more like 75%. ChromeOS is very popular in K-12 and macOS is decently common in University/Tech and not unheard of for the average consumer either. Traditional Linux is still marginal. This is only counting traditional desktop computing.
[1] https://en.wikipedia.org/wiki/Usage_share_of_operating_syste...