Only if every car has only one person - once you start getting 2 or more people in a car, driving starts to win[0] (and that's discounting the heavy multiplier you get from things like coaches and buses.)
Right, but you're missing the part where in the real world, nobody on the plane is going to carpool with other passengers they've never met, they're going to drive themselves in their own car, because people are not automatons that instinctively select the most climate-friendly option.
We shouldn't be comparing the most optimistic outcome in the world as you'd prefer it to be that could happen as a result of the flight being cancelled, we should be comparing the most probable outcome in the world as it actually exists.
> nobody on the plane is going to carpool with other passengers they've never met
I would put a good $25 on there being almost zero flights in the world where every passenger has no relation to every other passenger (to the point where you'd need N individual cars to transport them.)
I'd add another $10 on top to bet that there isn't a situation where all of those passengers would refuse to get on a train / bus / coach with the others if the plane wasn't available.
Those problems have never stopped the environmental movement before, so why would they now. (they could have supported nuclear a couple decades ago and we would be in much better shape environmentally)
The problem with the environmental movement is that it's not about saving the environment, it's about virtue signalling and chasing an internal locus of control over a phenomenon that is very strictly determined by factors outside of anyone's abilty to meaningfully influence.
