Do we have confirmation that he cleared the debts?
In any instance, I'm not terribly concerned. It's clear that this is a large project, and a Ligue 2 club is a very small investment. L'Equipe wrote that they were looking for €9m total for Valenciennes (including clearing all of their debts), and not all of that up front.
And from the Brentford - Mijjjjjjjjijijijijland experiment, it appears that the biggest benefit isn't acting as a direct feeder club, but rather then enhanced resources for player evaluation. A bunch of the Danes that Brentford acquired weren't from Mijiijijiijiland, but were almost certainly recommended to them by people at the club who were more familiar with the quality of player in the league. No team can have enough eyeballs on world football to get a good look on every emergent talent or underrated squad-ready player...but having a whole team's scouting resources at your disposal means a distinct advantage in scouting smaller leagues.
Given that Ligue 2 has for a long time featured a rich vein of young talent available for a pittance, that feels like an investment that could pay off even if Valenciennes never progresses up a league.