The Civil Service is not filled with "I told you so" Remainers, but in any case the mistake TTT is really making is thinking that the dogma is so one-sided. Those who have made their minds up, on both sides, will be unable to resist either ignoring any downsides of Brexit (Leavers), or drawing excessive attention to them (Remainers).
For a die-hard Remainer, the best outcome is to be proven spectacularly wrong. There is no joy to be gained from being even partially right.