I've seen the studies, and been somewhat surprised by their results, but I don't think they are an indication that fears are overblown, or that there's no such thing as filter bubbles. Rather, they indicate that the bubbles are not entirely about lack of information from the other side (although that is part of it). To stretch the analogy a little further, the bubbles we form around us exist to protect us from information from the "other" side. For instance, evidence that Twitter accounts from adversaries reference one another should not be taken as an honest exchange of information. These are not like references in an academic paper. From what I've seen on Twitter, my bet would be that the majority of these "references" are political opponents making snarky comments about one another. Not the same thing.
I fully agree with your point that there never was a time of "truth" for this to be considered an era of "post-truth" (I've written a Medium article about this — https://medium.com/@bigokro/pre-truth-politics-f370500a03fd). However, I think there is definitely a trend that has been going on for the last 20+ years towards "information tribalism" — self-imposed news and opinion filters.
The tumultuous Trump presidency to me is the clearest example of this. How can it be that a president so embroiled in scandals only varies in popularity by a few meager percentiles? How can you have something as clear, detailed, and public as the Mueller report, and yet have such black and white interpretations of it? We are so sure that the impeachment process will lead to a completely partisan result, independent of the evidence proposed. Why is that?
It does speak strongly to the possibility that if we're not in post-truth era, but rather a post-evidence era. That is, we have reached a point where attitude polarization is strong enough that a very small percentage of people are actually open to persuading by evidence. We know that confirmation bias is stronger the more you know about, or are emotionally involved with, a particular subject. So, perhaps the change is that in the information era, there are fewer people that haven't already been "tainted" by tribalism come election time.
One more cause for alarm, which has worsened dramatically since 2016, are the ways in which Trump has destroyed norms surrounding political and legal debate. His fundamental relationship with the truth is nonexistent — he gets away with saying what "feels true". Worse, he has demonstrated time and again that the right way to deal with any evidence of wrongdoing — accusations of sexual assault, obstruction of justice, "bribery" — is to publicly deny and deflect, discredit your attackers, and take it to the public. These tactics have worked so well (consider what Al Franken and Justice Kavanaugh have in common, and what they don't) that they might have become the "new norm".
So, maybe we never had the truth. But I think what's changed is that we no longer care.