People can choose to get their information from places other than the large platforms. To me, the "marketplace" is the internet as a whole. If people choose to stay on the platform, is it not similar to choosing CNN and not ever looking at Fox? In the end, people have many options but choose to limit which options they engage with

There’s a section in the EFF on why not being on the large networks is a detriment.

The usual idea of a marketplace is preserving people's freedom to go to alternatives. Yes, sometimes that freedom has to be preserved via intervention to preserve choices (e.g. antitrust laws), but it's always weird to me to see the critique used as a reason for less freedom rather than more.

Also the shock "wrongthink" things we see on the internet might not be a representative sample of what the general public actually believes.