Read the fine article. It explains in detail why "number of bugs" doesn't matter. Each successive version of Windows has an increased number of mitigation technologies, making bugs in general less exploitable.
That would mean "fewer known and unknown bugs" if successive versions of Windows were otherwise identical, and if the mitigations, being the only changes, didn't carry any potential to introduce bugs as changes do.
> It's very common to think about computer security primarily in terms of fixing vulnerabilities. In reality, security teams spend a lot of their time on a different goal: making bugs hard to exploit. This often takes the form of lowering privileges and introducing exploit mitigations. Windows 10 has a lot of investment in those areas, whereas Windows 7 doesn't contain any of the improvements made in the last several years. That's why even though Windows 7 continues to receive security bug fixes from Microsoft, it is considerably less safe to use.
Only if "fewer known bugs" means "more secure".