Nope, Liberum Veto existed, it just wasn't used. Parliament's acts were formally to be made by unanimous decision from the very beginning and some clever guy noticed one day, that if a single delegate was to block the proceedings, he can nullify the whole session. It's just that there was a certain sense of duty, that prevented this from being abused earlier. If someone didn't agree with the majority decision, he usually sat quiet, which of course doesn't mean that there was no hard political bickering earlier. Plus, the poor soul would be immediately threatened to recant his decision by the other delegates.