This is fairly new though, because what you said happened all the time and Youtube would unthoughtful act on them, without proof. People used it as a way to get people they don't like in trouble. And it is very hard to get the flags reversed. :(
But usually, a credit disclaimer is good enough for Youtube when it comes to games. If he used any music from any other person in his video, he wouldn't be able to upload it, even if he had a credit disclaimer. You have to have written permission from the original artist.
Copyright laws are confusing and contradictory. :(