“About this Mac” is not an application. It is a non-modal dialog box that does not stay open when you move away. What kind of use case do you have where you need it to stay open?
It makes sense now that you describe it as a dialog box instead of a "magic app that doesn't show up in the cmd-tab switcher", which is how I thought of it before. If it were any other app and I wanted to find the dialog again, I would cmd-tab to the app. I guess the problem is that whatever app it belongs to (Dock.app?) has no place in the cmd-tab switcher.
(on my Mojave machine, it does stay open when I move away from it, when I cmd-tab to something else and then a third something else, "About this Mac" is still visible.)
