A journalist ALWAYS needs to write something that is SOMEHOW different from what the reader believes. (If he's just reinforcing what the reader believes, why should a reader bother reading his output?)"
Actually, studies have shown that people tend to read authors and publications that tell them they are right. Echo-chambers existed long before the internet. So, while you ask why a reader would read that which reinforces his beliefs, the reality is he does.
Echo-chamber yes. But needs some difference, also yes. Even an echo-chamber medium is about giving the reader some new aspect to consider, new argument to use, etc. It may be primarily reinforcing, but it also adds or tweaks aspects to deepen the conviction and/or warp it into slightly better conformity with the common ideology of the journalist's in-group.
So I don't think there's really any conflict between our claims.