Sam2: I think that EVERY Rav has an obligation to say something if he feels it is halachically necessary, no matter how big or small he may be. A Rav also has a responsibility to know when he needs to ask another Rav for a second opinion.
As I said before, I think the letter was ambiguously worded, and I think everyone is reading it in a way it wasn’t intended. I wonder if a chareidi Rabbi wrote such a letter, would they be jumping to attack him, or would they try and find a limud zchus, such as reading the letter the way I did?