I'm not sure why you felt the need to create a new topic about this. You've already asked numerous times about this song.
The song doesn't convince everybody. Cera doesn't believe it from the start, as she points out that Grandpa Longneck said the story happened a long time ago. Petrie then immediately agrees with her, but then again it's arguable that Petrie is willing to agree with just about anything that the others think. Ducky may have been the only one that stuck with Littlefoot's belief that Doc was the Lone Dinosaur, and who knows what Spike was thinking.
The whole point of the song was to show how stories are usually passed down from generation to generation. Grandpa Longneck told it to Littlefoot and his friends, so Littlefoot decided to tell his own legend to the his friends. Also, he could have been overly excited with the thought that Doc may be the Lone Dinosaur which would cause him to add certain things into the story, or maybe these were things Grandpa Longneck told him off screen. We don't really know.
I think you might be over analyzing the song a bit. Yeah it's not the best song in the franchise, but I think it's a pretty harmless song meant to show Littlefoot's fantasizing over what the Lone Dinosaur is.