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].
[0] https://nypost.com/2019/06/24/us-holocaust-museum-denounces-...
[1] https://www.timesofisrael.com/us-holocaust-museum-rejects-na...
[2] https://www.jta.org/quick-reads/us-holocaust-museum-reiterat...