True, if the player in question isn't going to improve to any great extent. But they bought Yearwood when he was 19; Brentford was/is expecting him to get better over the course of the 4 year deal.
But you're right, there could be a million reasons that we don't know about.
Ideally you buy a guy and he plays 12 seasons for you. But young players are investments. There isn't any public information on what he transfer fee was to Brentford, but hypothetically if they bought him for $500K and then sell him for $1.8M then that's tidy business as well. They could need the money for other positions.
I mean if this guy went to Brentford and shit the bed so badly that they just want to get rid of him, it's hard to believe that Thelwell:
1. Wouldn't be privy to that
2. Would pay $2M for him