Antivirus software is a hot topic in IT security right now. Not because you need AV, but because most AV is terribly designed and breaks security in other applications. And while Windows Defender may not score particularly well on canned tests used by AV reviewers, it doesn't break as much software as other AVs do.
Remember that in order to work, AV has to inject itself all over the place in your system to intercept network activity, disk activity, etc. But if it does that at the expense of other security measures, is it really helping? As Justin Schuh said in his linked post, when Firefox implemented Address Space Layout Randomization (ASLR) to guard against buffer overflows, lots of AV suites disabled it by replacing Firefox's DLLs with their own which didn't feature ASLR. This stuff happens all the time, because AV vendors are always behind the curve in browser security compared to browser developers. Which isn't all that surprising if you think about it.
The upshot is, all AV software is pretty terrible. MS Defender isn't as good as some other AV suites at passing the canned tests that AV review sites throw at them. But at least it doesn't work against web browsers' built-in security measures.