The political content published in lemmy.ml isn’t the main problem, it’s the fact that lemmy.ml moderators are banning users from general interest communities, including non-political communities, when they criticize China or Russia elsewhere. This happens not only when the criticism is posted within a lemmy.ml community, but also when the criticism is posted outside of lemmy.ml.
The behavior of these lemmy.ml mods corrupts the “marketplace of ideas” that you’re describing. lemmy.world should not present or promote general interest communities that only allow users to participate if they avoid making comments critical of China or Russia on the fediverse. This kind of bad moderation creates a chilling effect.
While you certainly are free to not care about lemmy.ml mods blocking users from unrelated communities for criticizing China or Russia anywhere on the fediverse, which effectively silences lemmy.world users, there are currently 1,019 upvotes vs. 92 downvotes on the “Lemmy.ml tankie censorship problem” post, which suggests that most lemmy.world users who are aware of the issue do care.
Nobody is claiming to be trying to change the minds of lemmy.ml mods. We just don’t want to have to worry about whether posting something mildly negative about China or Russia on a post anywhere on the fediverse that somehow catches the attention of lemmy.ml mods will cause us to lose the ability to participate in a community such as !linux@lemmy.ml.
Condoning this censorship from lemmy.ml corrupts the “marketplace of ideas” because it distorts the discourse visible on lemmy.world to favor the political preferences of lemmy.ml’s mods. This affects every lemmy.world user, whether they block lemmy.ml or not, because there is no way to opt out of the chilling effects other than defederation.