The actual information comes from Microsoft's CVE listing [0]. Allegedly it's to prevent a privilege escalation to SYSTEM via the Windows Update stack.

(From attackers who can already add subdirectories to the system drive, I guess. Though I wonder why Windows Update would even be writing stuff into the inetpub directory in the first place.)

[0] https://msrc.microsoft.com/update-guide/vulnerability/CVE-20...