my experience with the splash mountain single rider line is that the "secret" was out many, many years ago — though I'll readily admit I haven't tried to use it since COVID. and at any rate, groups have been using single rider lines since before Radiator Springs Racers ever opened.
and at any rate, it's not really suppose to be a super-secret shortcut to the front of the line. it's a way to maximize capacity with the (hope) for a slightly shorter queue.