It's difficult to separate the two points: yes, tribalism and shared belief structures can and do affect perception. And yes, mass communication makes it readily more apparent. But that's not the entire story as the same communication developments have also given rise to the ability to more or less choose your own, more narrow micro-tribes (for lack of a better term) and effectively self-isolate yourself from opposing viewpoints and undesirable evidence. Cognitive dissonance and other cognitive biases can take it from there.
In the past, while your own biases could filter how you perceive contrary information, it was very difficult to outright avoid all contrary information (unless you just walked away from any news altogether). Now, we're getting to the point where that's become more and more of a new normal, while simultaneously allowing people to state with no hesitation that they're well-informed and pay attention to current events. It's just that they're paying attention while standing in their own narrowly-defined echo chambers that can become more and more isolated with time even when they know better.
Case in point? On election night back in 2012, Romney's team and advisors were surprised by their loss in line with pre-election polls that various conservative commentators rejected for a litany of reasons. I can see voters being surprised after buying into a certain spin, but the professional campaign staff with years of professional experience with that exact issue making the same mistake? Or what about the anti-vax movement or climate-change denial? Far from convincing people, overwhelming scientific evidence just serves to get them to double-down on their position.
None of this is new. Isolating yourself from new, uncomfortable knowledge has always been possible. But it was never as easy as it is now. And that's the point I think the parent was trying to make: the ease of isolation, and the extent of its spread across the politically active populace, have radically changed the negative consequences of the phenomenon at a societal level.
The hardest thing is that we aren't talking about something--the internet, social media, etc.--that's the cause here. It's not. A town's only neo-Nazi can go online to talk to like-minded shitheads to avoid having to painful answer that maybe there's a reason that they're their town's only neo-Nazi. The same tool can also help an LGBT teenager try and escape bullying by connecting with others with shared experiences who can tell them "It Gets Better." Being able to connect to like-minded groups isn't the problem; it's what happens when you do that, and then proceed to filter everything through your shared beliefs. Even when it isn't actively divorcing you from reality, it can still distort your perception.