From that PC Mag piece:

Microsoft Defender’s own developers seem to consider it a Plan B, rather than a main solution. If you install a third-party antivirus, Microsoft Defender goes dormant, so as not to interfere. If you remove third-party protection, Defender revives and takes up the job of defense again. The best antivirus programs, even free antivirus tools, perform significantly better in testing and offer more features.

Remember Windows Vista? Microsoft planned to secure Vista against the plague of viruses that began to appear with XP. But by then the Antivirus industry had come into existance - it would have stopped their AV programs working and put them out of business - and they hit MS with antitrust suits.

In another change, Microsoft had planned to lock down its Vista kernel in 64-bit systems, but will now allow other security developers to have access to the kernel via an API extension, Smith said. Additionally, Microsoft will make it possible for security companies to disable certain parts of the Windows Security Center when a third-party security console is installed, the company said...

[From Microsoft changes Vista over antitrust concerns]

Windows was made more insecure by design under the influence of AV vendors. And it's still like that: Microsoft maintains an uneasy truce with AV vendors.

In a well designed secure operating system 3rd Party AV programs like Norton should not be able to work. Apple doesn't allow it!