Getting banned for different views (assuming they were presented in a civilized manner) creates echo chambers. Surpressing other opinions will uniform thinking and perception. That enforces echo chambers.

Sure, Lemmy leans a lot more to the left due to it's user base. That doesn't necessarily mean it's an echo chamber as a whole. You can easily evade snowballing effects here by sorting your posts differently, e.g., scaled. There is no algorithm which tries to keep you on the platform by serving you stuff you like and hiding stuff you don't like. Thereby again, defying what's typical for echo chambers.

You can voice your views and opinions and sometimes surely get downvoted to hell for that. But as long as it doesn't break any "be nice" rules it's usually going to be still visible and seen by others. People might disagree, but you are not surpressed. Thereby again, not an echo chamber.

It becomes problematic though if specific instances or communities ban you for having a different (harmless and civilised expressed) opinion. That will indeed create echo chambers as only that is allowed which is similar to the common tone.
And admittedly, having a user base which is dominated by politically left leaning users, seems not to be helpful in terms of diverse political discussions (although I don't see how conservatives have ever been objectively or ethically right about something :p ), but from my experience this hasn't become a problem here, since even left folks love to shoot each other in the feet and have diverse and deep (often fruitful) discussions.