In Star Plus Mahabharat, season 3 episode 2, when Karna was a teenager, he wins a little competition which involved bow and arrow. Karna won, but Bhishma only praised his chariot driver skills, and told Karna's father, "here, in our area, it is tradition that only Kshatriya can be archer. And tradition is Dharma".
Very soon, Krishna says (there's a 1-2 minute class by Krishna in every episode, who briefly talks about the main thing of the episode, in an indirect way, to educate people and relate it to current life): There was tradition that people worshiped Indra, but later they found Dharma inside that tradition by breaking that tradition and worshiping Gowardhana.
So clearly, Krishna wants to prove what Bhishma said is not always true.
Also, I'm not sure what Krishna said was a message by Mahabharat producers/directors/content team, or was it really written in original Mahabharata.
So, was Bhishma really unaware of it?