It's not for the outcome, although the outcome is clearly a function of the type of position the defender went into.
The red is for a challenge that is dangerous, that is what the rules are telling us. This means a certain amount of jeopardy is built into the rules when you lift studs or move towards the ball high, or lunging from a distance which is naturally less controlled.
It is not the outcome that is defining the red card, it is when the above actions are applied to an opposing player in the form of a challenge. So yes you can lunge in exactly the manner the defender did when nobody is around, because it's not a challenge.