Do you want sources for what they claim about the rebbe that he is Moshiach?
They have sources. Like in the Rashi in the second to the last passuk of daniel, the Gemara Sanhedrin talks about his name possibly being Menachem. And there are more sources that explain how he could be Moshiach, the real thing they are wrong about it is that they know it as a fact when it’s not – we don’t know.
It’s not kefira – it’s a lie. From a perspective, yes, it looks like kefira, who are they calling him Moshiach if we don’t know that he is? But you can’t say he is not ra’uy to be Moshiach, so the big mistake they are making is saying it as a fact when it’s not.