They were definitely still biased. The difference is that the opinion was built into what they covered vs what they didn’t cover. ABC has always been a left-leaning network, CBS a right-leaning network, and NBC somewhere between. So while they might cover most big things very much the same, sometimes what they chose to cover or how much to cover it would reveal their leanings.
That being said, they were much more in-step than our news networks are now.