Critical scholars say lots of things. They are interpretations. There is no evidence that John is fictional. As a matter of fact, many parts of John have been confirmed and corroborated.
- John the Baptist, of course, is historical and confirmed.
- That they call Jesus “rabbi” fits the era. It was in AD 70, upon the destruction of Jerusalem, that “Rabbi” was reserved for the formally credentialed
- Nathanael’s disparagement of Nazareth is true to form. There is no prophecy of a prophet coming from Nazareth, and it was not only a minuscule town, but one of moral ill-repute.
- John 2 mentioning a Cana in Galilee, in distinction from the one in Judea, is true.
- Archaeology confirms the use of stone water jars for ceremonial cleansing (Jn. 2.6) in this era
I could go on but you’re getting the point. There is plenty in John that has been proved to be historical. Of course we can’t confirm that Jesus actually said these things, but it’s a fallacy of ignorance to claim that a dearth of evidence means it didn’t happen.