Agreed, Luke is the only eye-witness to what happened to his father and come to think about it, Luke might not have been too compelled to tell everybody that he was actually the son of Darth Vader.
But I think we can rest assured that the destruction of the second Death Star was somewhat communicated by the Alliance to the rest of the galaxy and accepted as the real thing (regardless of the S.E. ending).
The other thing is that Kylo has somehow acquired the helmet of Darth Vader from the burial site on Endor, already this suggests that Vader wasn't killed in the explosion of the Death Star. Although we do not know (yet) what kind of story came along with the head, I'd like to believe that Kylo would have been able to figure out that it's the result of cremation.
It currently looks to me like some kind of loose end and I'll wonder if the upcoming film is really going to elaborate on this.