Where is the source for that? Did the BHT write it himself – in which case please find me a link – or does it come from the fictionalized account of the Rayatz?
See Beis Rebbi (footnote Beis).
And while the book was written by a Chabad Chossid, he wasn’t a Lubavitcher, he was a Kopuster, so the last mutual Rebbe we had was the Tzemach Tzedek.