What browser are you using?
It isn’t a destructive operation. “Quote whole post” inserts the quote at the current cursor position.
It usually doesn’t delete any text unless you select some text before pressing “Quote whole post”. Selected text will be replaced with the quote.
But you should be able to undo the replacement with the browser’s undo function. Well, at least on a decent desktop browser. AFAIK there’s no undo function on Android (Chrome). iOS should have a hidden undo function, but I’ve never tested it…