This is the second description in Ahmad’s statement quoted earlier:

(They oppose the Book)

For each of them opposes the Book.

As for his statement that they are:

(Agreed in opposing the Book)

This refers to their giving precedence to something other than the Book over the Book—for example, their own rational judgments, subjective intuitions, opinions, and the like. In doing so, they agree in opposing the Book.

Whenever they abandon adherence to the Book and the Sunnah, they inevitably differ, for nothing can settle disputes between people except a Book sent down from heaven. As Allah Most High says:

“Mankind was one community. Then Allah sent the prophets as bearers of good tidings and warners, and sent down with them the Book in truth to judge between people concerning that over which they differed. None differed over it except those who had been given it, after clear proofs had come to them, out of mutual animosity. Then Allah, by His permission, guided those who believed to the truth concerning that over which they had differed. And Allah guides whom He wills to a straight path.”
[al-Baqarah: 213]

And as Allah Most High says:

“O you who believe, obey Allah and obey the Messenger and those in authority among you. If you disagree over anything, refer it to Allah and the Messenger, if you believe in Allah and the Last Day. That is best and leads to the best outcome.”
[al-Nisā’: 59]

Allah Most High also says:

“O you who believe, fear Allah as He should be feared, and do not die except as Muslims. * And hold firmly to the rope of Allah all together and do not become divided. Remember Allah’s favor upon you: you were enemies, and He brought your hearts together, so that by His favor you became brothers. You were on the brink of a pit of Fire, and He saved you from it.”
[Āl ‘Imrān: 102, 103]