The "Hide username from edits and lists" is presented to users with the hideuser permission. This option is only presented (or enabled after T212391) if the user gets the preceding options just right.
- Editing must be checked
- Sitewide must be selected
- Expiration must be set to indefinite
This is a lot of the user to know in order to do what they are trying to do which is suppress the user. Effectively, this option is presented to the user backwards. The user is there to do a suppression, not a block.
Move the checkbox up on the page to just under the target ("Username or IP Address") field. The checkbox should appear to anyone who has access (and be enabled if they put in a username). If they select the checkbox, the Editing, Sitewide, and Expiration options should be set to what they need to be set to and every field below the checkbox should be disabled because none of them have an affect when usernames are being suppressed.