I think the mentioning of them 'coming back' simply states that any and all characters (that could,) may have been there more than just what the shows provide records of
That's possible, though I really hope you're wrong (no offense), seeing as how it'd be really depressing to hear, "And they will return someday, but you won't get to see it because it will happen somewhere in between two movies."
That "Big Longneck Test" song was strange. I have to admit, when I heard the Big Water music start playing in the background, I was like, "the heck is he gonna sing about? Big Big Big Big Fire Pit?" I'm not sure I liked the new lyrics. It sounded a bit strained. The rhyme sceme seemed a lackadasical, and Littlefoot kept having to sing "Te-est" as two syllables.