He is saying that many programs are written to use "free RAM", ergo unless defender is still using 2GB ram when your PC is at your RAM limit you don't have a problem. Every OS does it, let your OS manage RAM.
Unlike linux[1], windows task manager correctly shows "cached" ram as "free" ram. Therefore it's highly unlikely that the memory usage is from the OS caching mechanism.
As for the actual behavior of using free ram, what happens if there are two apps that try to use the same behavior? ie. you have windows defender and a DBMS installed, both of them try to use up all the free ram. In this situation, what makes you think the behavior of "using all the available free RAM" behavior of windows defender wouldn't push out the "using all the available free RAM" behavior of the DMBS", leading to worse performance?
Windows has an infrastructure to prioritize memory availability by process, and to notify processes when there is physical memory pressure so that they can act accordingly. I'm not sure, but as a first-party component I would assume that Windows Defender uses these appropriately.
That said, like most real-time antivirus Defender does feel that it is important to complete real-time protection scans and will sometimes do so at the cost of performance. The logic here is that it is important to complete these scans even under conditions of resource pressure, otherwise malware could just do things like cause high system load before downloading a second stage in order to avoid Defender completing a real-time scan.
Unfortunately this does sometimes cause headaches, for example I saw a situation recently where someone ran a tool that opened a huge number of media files on a NAS in order to read their metadata. This resulted in Defender queuing up a real-time scan of probably over a TB over the network since it saw all of these 10GB+ files being touched, with a definite negative impact on performance. I still wouldn't give "exclude network mounts" as general advice as some people do, but that's an example of sort of a pathological case for real-time scanning where you probably want to exclude it.
Only the kernel should be grabbing "free" ram and allocating it for cache.