I think “security through obscurity is no security” concept was aimed toward people not relying on obscurity alone as a security mechanism. And largely that message succeeded. But now we are in a rapid acceleration of capabilities (on both sides) where any advantage to one side will result in outsized gains, at least in the short term.
And basically all the security bugs I've read about were find looking on the source code.
But it doesn't mean windows is more secure, just image a scenario where someone is stealing windows source code and sell it to rogue actor, it will make it even less secure because no one (expect windows) would have had the chance to search for bugs in the source code.
LLMs can read assembly better than most, so probably not. But reality has never stopped people from trying to obfuscate.