I see the same as you. The Registry key is a marker of the UI control not an alternative method of control. I was only after a marker so I got what I needed.

In How to enable Windows Updates for other MS Products for All users - TechNet I found a reference to a PowerShell script for controlling this setting [but the only script that had been referred to in that discussion had been a VB script].

@Kari includes a PS command for making this setting in para 2.1 of Update and Upgrade Windows 10 using PowerShell but when I tried it the script changed neither the UI checkbox nor the Registry key [despite showing the same responses as Kari illustrates].