Having everyone see the same news didn't mean there was no dissent and no discussion.

The facts shouldn't really be all that controversial. A quote from a political leader is a fact. Everyone sees this quote. People have different opinions about what the politician said, feel different ways about it, talk about whether they actually trust that politician.

Now with more "diverse sources" those source often decide to report or not report on something depending on whether it fits a narrative they are promoting. The alternative sources decide what people's opinions should be then determine which facts should be reported that align with those opinions.

The existence of these alternative sources allows people to choose sources that align to their feelings and never be challenged by inconvenient facts. A mainstream source that reports the facts regardless of whose politics it helps or hurts is seen to be biased relative to one's chosen source that always conforms to how they feel.

There is more groupthink now because people are never challenged with inconvenient facts. Sure there's multiple groups (that hate each other) but people within these groups have less real discussion and conform to the group more because they never get information that challenges how they think.

Most facts aren't really controversial. Ship loses power and hits a bridge. Bridge collapses. Poltiician says X in response. These are things that happened. Why would there be a variety on how this story is reported? It's only if there's a need to push an agenda that there would be diverse sources for this story. And most news stories are actually like this.