The ECF’s differed on many things, including the meaning of Matt 16:18.
Did any of the ECF’s have a “Protestant” understand of Matthew 16:18? I found these, and they all sound pretty Catholic to me:
Cyprian :“Our Lord, whose commands we ought to fear and observe, says in the Gospel, by way of assigning the episcopal dignity and settling the plan of his Church: ‘I say to you that you are Peter, and upon this rock I will build my Church, and the gates of hell will not prevail against it. And to you I will give the keys of the kingdom of heaven; and whatever things you bind on earth shall be bound in heaven, and whatever you loose on earth, they will be loosed also in heaven’” [Matt. 16:18-19]. From that time the ordination of bishops and the plan of the Church flows on through the changes of times and successions, for the Church is founded upon the bishops, and every act of the Church is controlled by these same rulers. Since this has indeed been established by divine law, I marvel at the rash boldness of certain persons who have desired to write me as if they were writing letters in the name of the Church . . ." (Epistle to the Lapsed 33

27):1 [A.D. 250]).
Cyprian: “On him [Peter] he builds the Church, and to him he gives the command to feed the sheep [John 21:17], and although he assigns a like power to all the apostles, yet he founded a single chair [cathedra], and he established by his own authority a source and an intrinsic reason for that unity. Indeed, the others were that also which Peter was, but a primacy is given to Peter, whereby it is made clear that there is but one Church and one chair. So too, all [the apostles] are shepherds, and the flock is shown to be one, fed by all the apostles in single-minded accord. If someone does not hold fast to this unity of Peter, can he imagine that he still holds the faith? If he [should] desert the chair of Peter upon whom the Church was built, can he still be confident that he is in the Church?” (The Unity of the Catholic Church 4; first edition [A.D. 251]).
Ephraim:“Simon, my follower, I have made you the foundation of the holy Church. I betimes called you Peter, because you will support all its buildings. You are the inspector of those who will build on Earth a Church for me. If they should wish to build what is false, you, the foundation, will condemn them. You are the head of the fountain from which my teaching flows; you are the chief of my disciples. Through you I will give drink to all peoples. Yours is that life-giving sweetness which I dispense. I have chosen you to be, as it were, the first-born in my institution so that, as the heir, you may be executor of my treasures. I have given you the keys of my kingdom. Behold, I have given you authority over all my treasures” (Homilies 4:1 [post A.D. 338]).