Mark,
“Show All Help Topics” does not “show all help topics.” It shows all help topics related to the text typed in. This is actually a list of “related” things. There are some apps that actually display all help topics when “Show All Help Topics” is chosen. I would think the menu item would be more accurate if it said “Show All Related Help Topics.” I have seen both ways done in apps but adding the word related is more accurate.
Occasionally I come across a help menu items where “Show All Help Topics” does in fact show all help topics. Although they have all been apps with a small amount of help information. A few have said “Show All Help Topics” and when selected went to a page on a web site with help information that have links to find more information.
There is an argument to be made for using both “Show All Help Topics” and “Show All Related Help Topics.” But a lot of arguments for “Show All Help Topics” I’ve hear seem to revolve around that’s what others do.
The last human “interface guidelines” Apple produced in PDF in 2014 doesn’t mention this.
The link to the current “human interface guidelines” doesn’t seem to mention it although it is now a pain to search and harder to be sure.
https://developer.apple.com/macos/human-interface-guidelines/overview/themes/
The closest the “human interface guidelines” come to this is saying “Avoid using the Help menu as a table of contents for your help book” and to “put links to in the help book not in the help menu.” They don’t say much else.
Bill