That doesn't mean you can't save games and load them.
I just tried it: loaded CAR mod. Ran 5 turns. Saved game. Ran 1 more turn. Loaded game that was just saved. Back on previous turn, just as expected.
I expect that what he was saying is that the save game version using CAR is not the same save game version as with the UP, so old games saved with UP would not be openable by UP + CAR. Or something along those lines. This is not really much of a problem. In fact, I would be surprised if adding CAR to the UP would change the UP's save game version since CAR supposedly does not change any game play, so only changes due to the UP would be present in saved games anyway (whatever they may be).