I was glad when the club brought him back as some serious competition for Dest and a defensively more solid option. I'm not really impressed by Dest, but like Emerson is he is still very young and needs time to develop. I agree that players need time and if you believe in them you should at least give them a season. However, Emerson didn't make a great impression in pre-season and in the first 3 games (although gotta admit small sample size with little time) and this might have changed club's mind. Imo Mingueza is a solid back-up too and the club isn't really in a luxury situation where you have a rather expensive back-up that you could cash in on. Koeman and Alemany probably just didn't see a future as a longterm RB in him and in that case and club's current situation it was reasonable to take Tottenham's offer.
Of course the club could have sold him next summer too, but a lot can happen in a year. He could get injured seriously, lose value because of bad performances or the club just needs the money now. Let's say you want Haaland, you'll need the money once the buy-out becomes active and like seen this year the market starts rather slow and it often takes time until you receive solid offers. Everybody knew for months that Ilaix hasn't a future at the club, yet it took Leipzig until the last day to sign him as they lost Sabitzer to Bayern.