Understood as referring to importance and rank, Jesus had always been “first” before his cousin, John. That is, Jesus had always been “foremost in importance” in relation to John. And this is something that would be true - and would’ve been true for John to have affirmed - irrespective of whether Jesus was alive before John or not. Even some Christians who affirm the doctrine of the pre-existence of Christ have acknowledged this fact. Consider, for example, the following remark on John 1:15 from John A.T. Robertson in his commentary: “[Christ] had always been (ēn imperfect) before John in his Pre-incarnate state, but “after” John in time of the Incarnation, but always ahead of John in rank immediately on his Incarnation” (emphasis mine). As is clear from the quote, Robertson (like all Christians) affirmed the view that Christ was alive in a “pre-incarnate” state. And yet Robertson did not understand the sense in which Christ was “first” or “before” John to have pertained to Christ’s pre-incarnate state. Rather, he understood John to have been saying that Christ was “always ahead of” him (John) in rank “immediately on his Incarnation” (i.e., from the time that Christ entered the world at conception, and began life as a human).
John's Account, Part Three: http://thathappyexpectation.blogspot.com/2017/11/a-consideration-of-passages-thought-to_90.html