Step 5~6 still have problem, and the new added “Clear History dialog box” have similar symptom.

step a. Click “Clear History…” button. (The cursor should stay on the 1st item “Recently used files,…”
step b. Check/uncheck the check box on the front of the “Recently used files,…”
The [Enable/Disable All] check box will be synchronized…

Refer to help file for “Customize dialog box” and “Clear History dialog box”

Enable/Disable Selected Items check box
Enables or disables selected items on the list. 

Suppose current “Enable/Disable All” function, was “Enable/Disable Selected Items” function.
But help file did not change accordingly.