It isn’t as big as some other places, yes, but to say it isn’t one doesn’t really sound true either.
an echo chamber is an environment or ecosystem
in which participants encounter beliefs that amplify or reinforce their preexisting beliefs
In the months I’ve been on lemmy, not once have I encountered a right-wing post, yet I see left-wing posts multiple times a day. By definition, how is it not an echo chamber? I suppose lemmy as a whole might not be, but the resulting lemmy I browse sure is.