Kinda causes problems when someone who knew someone who knew an apostle tells you that the See of Rome is the touchstone of orthodoxy, huh?
Maybe most people within the church at the time didn't consider his claim to be particularly "outlandish". Maybe they considered it to be something closer to common knowledge. We certainly have no record of anyone at the time, especially anyone with a more impressive pedigree of Christian discipleship, saying, "Irenaeus, you ignorant fool! Whatever caused you to come up with such an outlandish idea?!?"
And just how would we know if that someone who knew someone told someone or anyone for that matter that the See of Rome is the touchstone of orthodoxy? Where is the documentation for that?
And how did Peter and Paul and John forget to put something so important into their epistles? Perhaps this is what Irenaeus means when he uses the word "tradition" --- a tradition of forgetfulness on the part of those chosen spirit-filled apostles that left room for subsequent generations to invent doctrines, create documents and make things up as they go along.