An echo chamber is not somewhere where everyone believes absolutely the same thing about everything.
But this forum is dedicated to a football club, which means that 99% of people using it are fans of the football club.
That football club is based in East Manchester, which means its fans (particularly older fans) overwhelmingly come from that area.
Forums are an old fashioned form of social media, which means the age range of users is definitely tilted older than reddit or twitter, but not too old because social media use among people 70+ is less than half that of 18-50 y/o people.
Football fandom is dominated by men, something like 80% of fans in PL stadiums are men.
Immediately if you're on here, there's a good chance you are a 30-55 year old man with connections to the North West of England. Which makes you likely to have the same views as other 30-55 year old men with connections to the North West.
Some people won't fit into that, and some people in that demographic will have contrasting opinions, but generally if you spend any time on the politics forum espeically (which I know you do) a consensus forms very quickly.
To bring it back to your definition of echo chamber -
In news media and social media, an echo chamber is an environment or ecosystem in which participants encounter beliefs that amplify or reinforce their preexisting beliefs by communication and repetition inside a closed system and insulated from rebuttal.
People from the same demographic tend to have the same or similar beliefs. This forum is repetition and communication. It's a closed system because only people who sign up to be a user of Bluemoon can interact (same way twitter/fb/youtube/parler onlyusers can interact).
And the like button is the perfect example of amplification. You think what I think! Like. Now everyone knows we think the same way, and more people who think the same will like it as well. 40 likes wow! So many people agree with me, I must be right!!