Windows Defender processes run with high privileges, and can be exploited (in theory) by the malware via Defender vulnerabilities. This is also true for any AV and can be very dangerous, so running those processes in the sandbox is reasonable. They still can be exploited, but the exploit is isolated from the system in the sandbox.
WD Tamper protection is another kind of self defense. It prevents other application from changing some WD settings (no exploit of WD processes). Microsoft does not say what settings will be protected, but I can guess that disabling WD or disabling WD real-time protection will be included. That is why, probably, Microsoft forced me to exclude 'disabling WD real-time protectionin' setting from ConfigureDefender.