While Meshichistim aren’t making a mistake. They’re in denial, and using crooked logic to justify denial. But would that be Kefira?
The Rebbe, according to any normal way of determining things, passed away. If he was married, his wife would be allowed to marry someone else, etc.
The only possible way I can see (please, if you can offer a reasonable alternative, I’d like to hear) for someone to think he didn’t die is for them to consider him immortal, i.e. not a basar vadam. To me, that’s kefirah.