The point Perry is that during the oral tradition period, well before the synoptic gospels were written, there were easily accepted alternative versions of who Jesus was and the good news he preached. Some of these different gospels had already been accepted in the Corinthian congregation Paul is warning. He says they were putting up with these different versions, proving that embellishment and corruption of the message was rampant very early on right in the thick of the congregations.
Remember, most of these early Christian communities in the far flung regions of the pagan Roman empire that Paul was preaching too were not literate people. They relied on oral traditions. They had previously believed in mythological deities like Mithras, etc, and they wanted their Jesus to be just as powerful and god-like as their former pagan gods. A Jewish vagrant preacher telling of peace and mercy who then dies on a stake didn't quite cut it for these pagans.
Christian apologists try to refute this by saying that the authentic oral tradition was protected from embellishment by the authentic Jerusalem apostles/disciples who had been eyewitnesses of Jesus being out travelling amongst these far flung congregations to act as a preserving influence against corruption of the true oral traditions. But 2 Cor 11:4 proves that this had not really occurred. No original Jewish eyewitness disciples were out there in Paul's congregations protecting the original eyewitness accounts, or if they were, they largely failed.
Christian apologists also assert that as the gnostic gospels were written after the synoptic gospels (except Thomas, arguably), this proves that the synoptic gospels are the authentic gospels. But Paul's statement at 2 Cor 11:4 proves that there was a marked tendency very early on for very different and easily accepted oral versions of Jesus and his message to develop and get preached.
In a nutshell, 2 Cor 11:4 shows that the oral period was fluid and rife with embellishment and variations from the beginning, and that these embellishments had got ingrained in the congregations well before the written accounts were recorded. In other words, the synoptic gospels cannot be trusted as written records of the original, authentic, eyewitness accounts. The original oral tradition is lost, and in any event were probably mostly 'Q' accounts that quickly got mythologised.