Everyone on the ISS needs to have a seat reserved for them in a docked spacecraft, in case they need to evacuate the station quickly (or for a medical issue like this). You can’t bring back just one person from a 4-person crew; the other 3 would have no way to leave.

Well, yes? The medical issue is apparently severe enough to warrant return. Because the crew dragon is the only way for those astronauts back, barring sending another one up shortly, they also have to come back.

The reason they are bringing the whole crew back is most likely cost related. The whole crew was due back in February anyway. They are bringing everyone home a bit early; otherwise they would need another flight a few weeks later.

And nobody is retreating: there will be 1 American and 2 Russians left on ISS. All of this from the article.

This mission doesn't matter to any normal human in any relevant way that NASA would need to hide anything.

I'm completely lost on your way of thinking

It's not that the entire crew is compromised medically, it's that logistically if one goes home they all have to.

Bruh, you're talking about one of the most protocol laden risk averse organizations known to man. That's an absurd speculation compared to the thing you would naively expect, which is exactly what is happening.

Why the conspiracy theory?