OP there is only Watchtower logic about your premise that if the flood did not exist then Jesus' words are false.
It is a scientific given that the Biblical flood could not have happened but quoting myths as if they are realities is the religious norm.
What we are dealing with here are words attributed to Jesus written way after he was supposed to have lived.
In the first and second centuries there were lots of christ cults, god-men and saviours. It was in the interest of the leaders of a christ cult to include in their text, the endorsement of Noah's flood by its particular god-man hero.
Bear in mind that the earliest non-canonical christian writings do not give the name Jesus but call him Lord. The name "Jesus" certainly appears by the third quarter of the second century according to Melito of Sardis however his works like all manuscripts and especially "christian" ones, were subject to being edited by later tamperers and zealots such as Bishop Eusebius of Caesarea. Manuscripts can be altered at each re-writing, to add a name or attribute a belief to a character was easy peasy--just make sure you destroy the earlier copies.
We have the Roman Catholic church at Emperor Constantine's urging to thank for the unification of all Christian (and pagan) cults under the name of Jesus in the fourth century. By then the idea that Jesus existed was not in doubt in the minds of the faithful. The poor people had no choice but to believe in authority.