Guirassy performed exceptionally last season, but not many clubs were interested in him, so Dortmund didn't sell him. It's different with Haaland and Bellingham, because both players had such opportune moments that many clubs were interested in them. I think because there weren't any big offers that matched Dortmund's wishes, they didn't let Guirassy go this season. If Dortmund received a big offer in the next transfer window, I'm sure they would let him go. If they were planning to develop the team, they would have already signed several players in the transfer window, but Dortmund didn't do anything.
Borussia Dortmund player transfers looks to depend on market. They kept Guirassy because no club met their asking price not because they were focused on developing team. This is different from their handling players like Erling Haaland and Jude Bellingham and they were sold because their amazing performances created huge demand and led to very high offers. Dortmund has not signed many new players means they are not trying to rebuild team right now. Instead of this they appear to be operating with business first strategy willing to sell Guirassy if they receive good offer in future.