The teachers Irenaeus wrote against claimed a tradition handed down privately to those capable of receiving it, alongside the ordinary teaching given to everyone else. His answer is procedural rather than speculative, and it is the reason the book mattered.
If the apostles had a hidden doctrine, he argues, they would have entrusted it to the men they put in charge of the churches, and those men and their successors are known by name and can be listed. He lists them for Rome, and points to Smyrna and Ephesus. What those churches teach openly, and have taught continuously, is therefore the measure. A doctrine that appears in the second century with no public line behind it is new, whatever it claims about its origins.
Alongside this he sets the rule of truth, the summary of belief a candidate receives at baptism, and uses it as a frame that determines how scripture may be read. His illustration is exact: a mosaic portrait of a king can be taken apart and the same tiles reassembled into a fox or a dog, and the person who has done it can still say that every tile is original. Heretical reading, on his account, does not add material, it rearranges it.
The same instinct produced his insistence that there are four Gospels and only four, which is the earliest surviving statement of the number, and his doctrine of recapitulation, that Christ passes through every stage of human life and repairs it from inside, with Mary answering Eve at the point where the damage began.