Dear Menachem,
The section of Kisvei HaAri that the letter quotes is the same section that has the erroneous passage discussed in the letter to Rav Zevin. Why the Rebbe chose to keep this myth somewhat intact, is a mystery. But his intent is clear. He is associating the day with Penmius HaTorah, as opposed to just the day Rebi Shimon died. Why there is a day for Penimius HaTorah is beyond my understandind. And I wonder if he really believed it to be. What the Rebbe is really asking for, is that we all commit to understanding chassidus and bringing it into reality.