Sure, I know about C-h b and C-h m and find those useful. But I think what the GP post describes is more contextual: A way to not display every keybinding, but only those directly accessible by pressing the currently held modifier combo followed by one key (so holding Ctrl+Meta for more than a couple of seconds might remind you of all structural editing commands for example).
This is indeed not possible in a classical terminal emulator (don’t know if kkp for example has extensions for it). But most GUI apps can detect individual modifiers being pressed and released, even as separate events. Some editors like VSCode can also bind modifier taps to actions using this ability. In Emacs however this is AFAIK not possible even in the GUI, because of how keybindings are handled deep down.
The UI pattern described by the GP does exist in some other apps and platforms. For example, if you connect an external keyboard to an iPad, holding down the Cmd modifier for a couple of seconds will show you a popup which-key-like overview of all Cmd+key keybindings.
