The credited writers for Flubber are John Hughes and Bill Walsh. Walsh was the writer and producer of the original Absent-Minded Professor and Flubber movies from the 1960s. Hughes simply rewrote Walsh's original screenplay and gave him a writing credit.
Walsh passed away in 1975, so he was not involved with the 1997 remake. The Critic may not have been aware of this when he tried to exonerate John Hughes from any wrongdoing and assumed it was the other guy (Walsh).