More than 400 Holocaust and genocide experts have asked the US Holocaust Memorial Museum to reverse its stance that comparisons of ICE "concentration camps" to those of the Holocaust are unwarranted[0].
This letter does not appear, at all, to endorse the silly use of the term "concentration camp" in reference to U.S. immigration detention facilities. Rather, they seem to have taken issue with one particular broadly written sentence fragment in the museum's statement:
“...unequivocally rejecting efforts to create analogies between the Holocaust and other events, whether historical or contemporary...”
The letter was written in the context of AOC using the term "concentration camps"; this is what caused the Museum to release the statement in the first place[0][1][2].
It doesn't really matter what context the letter was written in. It very clearly only takes issue with a particularized sentence fragment used in the museum's initial statement. Nowhere does the letter endorse or promote the idea that U.S. immigration detention facilities should be compared to "concentration camps."
You are claiming that this letter says something that it, quite literally, does not.
[0] https://www.nybooks.com/daily/2019/07/01/an-open-letter-to-t...