But getting back to the OP. How aren't you fascinated by the identity of Irenaeus?! He says in one paragraph in Book 3:
John, however, does himself put this matter beyond all controversy on our part, when he says, "He was in this world, and the world was made by Him, and the world knew Him not. He came unto His own [things], and His own [people] received Him not."(8) But according to Marcion, and those like him, neither was the world made by Him; nor did He come to His own things, but to those of another.
Implying that he knew the prologue to John and then a few lines later says:
For the Ebionites, who use Matthew's Gospel only, [Ebionaei etenim, eo evangelio quod est secundum Matthaeum solo utentes], are confuted out of this very same making false suppositions with regard to the Lord. But Marcion, mutilating that according to Luke, is proved to be a blasphemer of the only existing God, from those [passages] which he still retains. Those, again, who separate Jesus from Christ, alleging that Christ remained impassible, but that it was Jesus who suffered, preferring the Gospel by Mark, if they read it with a love of truth, may have their errors rectified. Those, moreover, who follow Valentinus, making copious use [plenissime utentes] of that according to John, to illustrate their conjunctions, shall be proved to be totally in error by means of this very Gospel, as I have shown in the first book. Since, then, our opponents do bear testimony to us, and make use of these [documents], our proof derived from them is firm and true.
There is no such 'only' in the reference to Marcion's use of Luke. Indeed many have noted that Tertullian (Irenaeus) in Book 4 says clearly that Marcion had all four gospels in front of him but chose to falsify Luke. There is always this 'door open' to other gospels. The idea is:
1. the Ebionites ONLY used Matthew
2. Marcion falsified Luke but knew other gospels (the textual criticism of Luke helps 'expose' Marcion's falsification but Marcion is not a one gospel heretic like the Ebionites)
3. this unknown group uses Mark in a 'wrong way' but no specific mention of falsifications is referenced. There is some implicit sense of exclusiveness (otherwise why must they be shown their errors with respect to Mark).
4. the Valentinians plenissime utentes again not exclusive and it is only to prove these aeons so the Prologue.
The truth is that there is nothing Valentinian about John other than the Prologue. A lot of the misunderstanding of Irenaeus is developed from laziness. The reason why the Ebionites have this exclusivity again has more to with propaganda. If Matthew is the first gospel it is at least theoretically possible that a Jewish Christian group would only know the earliest gospel because - supposedly - the Jews were first. Not worth taking seriously otherwise other than to figure out Irenaeus's lies and myths.
The unnamed communities exclusive use of Mark is far more intriguing. But the bottom line is again:
1. the Ebionites were first and used (what Irenaeus calls or assumes is) the first gospel = Matthew
2. the Marcionites may have been exclusive to their gospel but it is not exclusively Luke. Studying Luke helps expose their master Marcion's fraud apparently. But they never framed their gospel in terms of being 'Marcion's gospel.' It was a gospel of Paul and the many references in Paul to 'my gospel' and the like are used to support their understanding.
3. there is an unnamed 'Mark' community that implicitly uses Mark quite exclusively. Irenaeus says if you teach them to read Mark correctly their errors will be corrected.
4. the Valentinians were only interested in the Prologue and specifically for their aeons. Not married to any one text.