You raise very valid points. The case you describe would be characterised as “oppositional reading” of mainstream media by those believing in a conspiracy.

However, this is unrelated to the common understanding echo chambers. A big problem with the concept is that there is no robust definition, and the fact that it is used widely in mainstream discussion does not make it easier. The tendency to preferentially connect or communicate with like-minded people (homophily) is nothing new, we encounter it everywhere—think hobby groups, special-interest forums like HN etc. Social media’s affordances especially promote homophily. What the filter bubble/echo chamber concepts add to this preference is an exclusion of outsiders. The above commenter summarised a purported consequence as “you’re sucked into an echo chamber with only news that confirms your views”. In this sense, using diverse news would be incompatible with an echo chamber.

What is especially interesting, then, is that ideological polarisation is very much alive on the internet, despite (or indeed because of) the absence of echo chambers.

I thoroughly recommend the article I linked above. It touches upon these concepts and cites current research that allows you to dig deeper.