A much better translation would be 'Faithful to The Rebbe Mh"M.'
If this is what he was talking about, "faithfully believed in the Rebbe's messiahship" is a horrible mistranslation.
I'd also be interested to see the original Hebrew it is translating, it could be a misunderstanding/mistranslation.
Though, that was before I realized who the author was. If this is the kind of thing he is basing it on, it's not a mistranslation, but a willful misrepresentation. Aka, a lie.