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excuse for why he can't do it (despite Gordon Brown holding a position while still an mp) whereas in fact the offer was withdrawn on account of him being both useless and a massive cunt.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)
Apparently they offered him the job thinking he'd be resigning as an MP to work for them full time. And he's not.
What a twat for thinking such a role could be part time.