Let's be honest, who is actually willing to take the time and effort to read the exact same news from every possible news source to determine the legatimacy of all of the articles' claims, "facts", and opinions. What these kids are likely doing is spreading themselves out, covering all of the sources, and coming back together to discuss them; better than reading one news source, but also worse than reading all of them. At least that's the perspective that I get from the author's writting.
Judging by the handles of these accounts it wouldn't surprise me with kids only followed accounts that aligned with their current beliefs which may end up leading to the echo chamber you previously mentioned (however I must disagree that it's not the direct copy and pasting that makes it an echo chamber, its the copy and pasting of ideas). But also because of the online and pseudonymous nature of the internet it also wouldn't surprise me if people also joined communities that didn't align with their beliefs just to experiment with them. Perhaps even engage in more civilized debates on Instagram than in /r/the_donald.
But also as an occasional user of Instagram I find it weird for them to even be having long winded debates on the app. It was designed to be a photosharing app with comments, not a place for discussion threads. But maybe I'm too old now ️