Isn't that more of an urban myth ? I mean, amusing, but not really the case.
He declined to extend his contract and left by mutual agreement at the end of the 2005/06 season.They finished 13th. The next season they were 19th and relegated. (https://www.transfermarkt.co.uk/charlton-athletic/platzierungen/verein/358).
Season before his last they were 11th and season before that 7th (the only time in the top 10 bar 00/01 season (9th)) and season before that 12th.