By Rav Shach, you wouldn’t think twice about him being Moshiach.
It’s very different by the Rebbe.
I think you understand why. If you don’t I’ll show you in short.
One of the things necessary to be Moshiach is that this individual is passionate & his whole life is based around bringing the geulah.
Rav Shach’s whole life did not revolve around bringing the geulah.
The Rebbe’s whole life did revolve around bringing the geulah.
But, that doesn’t make it stamped in ink. That doesn’t MAKE him Moshiach. But it would make you think twice.