But the comment was a generalisation in regards to what the OP was doing, and I think we can assume they aren't snapping on / off the throttle / brake. Plus the guy driving in that video admits to driving an unstable car and presumably had no issues when the spoiler was installed.
Obliviously you can drive in a straight line wrong, hence the inclusion of the word "really". The same way you can eat a sandwich wrong, but not really, and you have to be doing something silly.
However, to clarify the intended point of my statement - It's hard to drive incorrectly in a straight line at constant speed. If all you are doing is trying to keep the car straight, it shouldn't feel unstable, and if it does, it is most likely due to the car not the driver.