MacOS 10 wasn't rebranded from OS X until 2016, a full year after Windows 10 GA - and I doubt anyone outside a select few at Apple knew that the OS X line was going to be renamed.
In any case, your reasoning doesn't really make sense. I can run a program that was written for Windows XP on Windows 10 without the need from a app-compat layer. Given that a developer can hide/show all sorts of random functionality with an if-branch-on-version - the user will see a broken or strange app and it won't be clear (and MS probably didn't have the means to detect) that the app should run in compatibility mode.
I still believe that MS wanted to ship an OS that "just worked" and did so under 10, than trying to compete in version numbers with an OS that has had 10 in its name for last 18 years.