Shaun,
Its the air movement that dries the carpet/ upholstery out, the dehum takes the excess moisture out of the air.
It takes a while for a dehum to settle down and start to work, it wouldnt work in our time parameters. I have put a building dryer in a flooded lounge and dried an Axminster out because the Insured said the carpet wasnt made any more so wanted it dried out rather than be replaced. That was over a few days, dried the carpet and the underlay out a treat.
If your not sure do the BDMA flood course or the Flood School.
Regards
Murky