Nope, it's not a KASU because there really were supposed to be 5 Megamorphs. The 5th Megamorph was supposed to be the series finale, which would have featured a point of view perspective from all of the characters like all the other Megamorphs, and it was supposed to be released at the same time as #54. However, it was scrapped at the end because Scholastic felt that releasing both #54 and Megamorphs #5 at the same time and marketing them both as the final book would've been a little confusing for the kids, so K.A chose to only make #54 the last book instead, which is why it plays out like a Megamorphs book with perspectives from different characters.
The contract for Animorphs still held for a book to be published for the series, so they planned on replacing the 5th Megamorph by publishing a series bible instead. That for some reason also didn't go through as a book, but was instead turned into the Anibase that we have now (It's the same information that would've been released with it). K.A at the time was also writing Everworld which was supposed to have 1 extra book in the series according to its contract, so what Scholastic and K.A ended up doing was that they moved those two unused books from the two series into the Remnants series, giving it an additional 2 books.
Actually, it might have been 3 extra books in Remnants altogether, because K.A had also planned on writing a Taxxon Chronicles as well. It would have featured Arbon's storyline, but K.A ending up scrapping that because she chose to add him in #53 instead. I just can't remember whether the Taxxon Chronicles turned into the planned 5th Megamorph after it was cancelled, or whether the contract added the book for that into the Remnants contract like the others as well.
