That's last year's MQ and is a rating for commercially available add-on products.
I work in cybersecurity and what you need to know is that Gartner is ranking products based on features and capabilities that are important to enterprises. So it's more than how does it do protecting against viruses and malware. The much better judge of a solution like endpoint protection is how does it protect when measured against things like the MITRE ATT&CK framework. Microsoft Defender ATP which is only available to enterprises (and what was used for this ranking), is different and contains a lot more capabilities than Windows 10 Defender. I think in our internal testing Defender ATP came in second or third and the other two vendors in our top three are in Gartner's lower left and lower right quadrants. Also, one of the vendors in the top right quadrant was ranked at the bottom when we tested it. So I wouldn't take everything Gartner says as canon.
When it comes to personal computing protection there are much better solutions than Windows Defender. I couldn't tell you as I don't do Windows . I'm a macOS and Linux guy.
We're drifting a ways off topic but absent other published results by reputable independent research firms Gartner is a good base to build on. My point wasn't specific to any version of Defender, but rather that Microsoft knows what they are doing in this space (and if one is being thwarted by Defender there's usually a good reason why).
https://www.techrepublic.com/article/why-microsoft-spends-over-1-billion-on-cybersecurity-each-year/
I'd be very curious to see independent results point to better-performing security solution for Windows.