The term is a euphemism because "concentration camp" just means a camp with a high density of people. The fact that people know what they're really referring to doesn't mean that the term isn't a euphemism. After all, the whole point of a euphemism is that people know what is really meant.

It's quite well known that "concentration camp" is not the correct term for the Nazi extermination camps.

Just like it's quite well known that "hacker" isn't the correct term for someone who breaks into computers.

All that remains is the small detail of convincing the public at large that they are using the word wrong.

As I said, it's pretty well known that Auschwitz and the other camps like it were extermination camps, not (merely) concentration camps. I mean, I'd count myself a member of the "public at large" in this context (I'm certainly no expert on the Holocaust or the surrounding history), and yet I still know what the correct term is. Anyone who has so much as looked at the Wikipedia article for Auschwitz or one of the other camps will know the correct term.