Lydia McGrew wrote a good article earlier this year making that point. The passage implies that John the Baptist was born before Jesus, which agrees with Luke's gospel. John the Baptist's comments in that verse would make less sense if he was born after Jesus. The context in John 1 is about Jesus' deity and preexistence, his existence prior to his life on earth. If Jesus was born before John, then John's comments make less sense, since, under that scenario, Jesus' existing before John wouldn't imply anything significant in the context of John 1's emphasis on Jesus' deity and preexistence. By contrast, if John was born before Jesus, yet John says that Jesus existed before him, that has a lot of significance. So, John 1:15 makes the most sense if the passage is agreeing with Luke's gospel about the order in which John and Jesus were born.
For more regarding what John's gospel tells us about Jesus' childhood, see here.