Kinda depends what is encrypted there. If it is just "magic number + SKU + licence ID" and there is no online check whether that combination is valid then you're "just" trying to hit one that's valid and that cuts few bits off equation as there is spectrum of keys that will be valid but not generated by Microsoft.

So less possible combinations.

Which doesn't mean you shouldn't also use obscurity. NIST recommends it [1], and the industry widely uses it. In practice "don't rely on obscurity" usually means "have enough security besides obscurity to give you a grace period to switch out the obscurity". That's for whole systems, you might get away with people knowing you use standardized primitives like AES.

[1]: https://csrc.nist.gov/news/2021/revised-guidance-for-develop...

if it's not obscure enough that it will be found, then you are correct.

but what's more secure than a password that you don't know? not knowing there is a password in the first place. if the answer is never found, how can it be insecure? I dub this schrodinger's security.