The definition of mu‘allal mentions both chain and text because a hidden defect may occur in either.
Keep the distinction clear in this example:
• The particular chain: it is defective because the wrong narrator is named.
• The Hadith text: it is sound in this example; the error in this chain does not make the text erroneous too.
A defect in one chain does not necessarily mean that the same wording has no sound transmission.