I think he has used that as an excuse for why he can't do it (despite Gordon Brown holding a position while still an mp) whereas in fact the offer was withdrawn on account of him being both useless and a massive cunt.
(it may have been phrased slightly differently)
Matt Hancock has had his UN job offer removed and this quote
“The last thing the African continent needs is a failed British politician.
“This isn’t the 19th century.”