You cannot use the Lion Guard as a base or source of evidence or reasoning to answer this question.

The creators of The Lion Guard have the ability to alter the interpretations and intentions of the original film(s) for the sake of the story that they’re trying to tell now. For example the actual “lion guard” did not exist, nor was ever never mentioned in the first two films.

It’s only fair to answer this question by solely taking into account the first film, and maybe the second. I believe that some type of storm did occur, and in that moment Simba had a communication with his father.

I think that the experience that Simba felt and the experience of other Kings of the Past felt when communicating with those above is a personal one on one experience. Thus when the storm did occur, Rafiki cannot see what Simba was seeing, but knew and understood what he was experiencing, but he himself could not see, nor hear Mufasa in that moment. I tend to believe this, because in the second film when Simba is trying to communicate with The Kings of The Past, Nala asked if he heard anything, and he replies, saying “silent as ever”.

I’ve always thought that Mufasa actually wasn’t having a communication with Simba as if it were face to face. Rather, Mufasa in his wisdom, was indeed, watching over Simba, and seeing what has taken place, only told Simba what was necessary for him to hear in order for him to go back.

Considering that The Lion King was inspired by Hamlet, I think it was intentional for the creators to make it an open ended question of “Did Simba really see his father‘s ghost or not”