In The Scarlet Pimpernel why is it ironic that Bibot thinks he will be the one to capture the Scarlet Pimpernel?

The irony in the expectations that Bibot has of catching The Scarlet Pimpernel is that despite his intense efforts and close eye, the Scarlet Pimpernel escapes, right underneath his very nose.  Bibot prides himself on his vigilant capture of any escapees; he is confident that the elusive and enigmatic Pimpernel will never escape his clutches.  The irony is that even as he is bragging about this, the man himself slides right on by, without Bibot even suspecting.  The Scarlet Pimpernel did have an ingenius disguise; he was dressed as an old "hag," or woman, with a bundle of diseased joy in the back of her cart.

So, the joke is on Bibot; his ego is taken down a notch, and the Pimpernel wins yet another victory on Bibot's watch.  I hope that helped; good luck!