Tbh yes it was poor management by Matton, but Andreas was always going to go to Hyundai over Citroen by all accounts. He went to Hyundai because he couldn't really drive the C3 and he felt he could challenge for the championship in the i20 and a car he felt at home in and quick.
Matton should've done more about looking outwith Ogier, but more in terms of going for a young up and coming driver or trying to get Sordo/Paddon to help me the team out for 2018. Mikkelsen was only going to Citroen if no other team offered him a drive for 2018 and Matton probably knew that.