Cocoa Touch, while being slightly simpler and less arcane than Cocoa, still has a lot of semi-hidden behaviors that are nonobvious to cross-platform UI framework implementers.
I'm trying to find an example of this "long press shows context menu". Tried long pressing a link in Safari Mac no context menu appears. Ctrl-Click brings up context menu. Tried long pressing a shortcut, no context menu but ctrl-click brings up context menu. Tried selecting and long clicking a file in Finder. No context menu appears but ctrl-click brings up context menu. Tried selecting some text in TextEdit and long pressing, No luck. Ctrl-Click brings up context menu. Tried long pressing a tab in the ruler, same issue. It seems like even for Apple this isn't much of a standard.
The person to whom I was replying was talking about Cocoa Touch, so this was about iOS, not macOS. If it interests anybody, the specific example was Pokémon Go's text fields which _do_ respond to Cmd-V, but I can't long press to get Copy|Paste to show up.