It means they're capable of burying news stories that would contradict your worldview and pushing news stories that support your pre-existing biases, leading to more engagement from you (a win from their point of view) but also burying you in an echo chamber. And unless you were in the habit of doing the occasional search in Incognito Mode, you wouldn't know. (And even then, they probably would be able to put together enough clues to figure out your identity even without your Google login cookie).

Yes, the fact that they could do it does not prove that they were doing it, not right away. I ditched them as soon as I found out that they could do it, because I was absolutely certain that eventually, they would end up doing it. And I wanted neutral search results, not biased ones, even ones biased towards my own point of view.