Because he played for Bielsa at Bilbao. Bielsa is Pochettino’s mentor. They both eschew a similar high-octane game with focus on getting the ball forward as quickly as possible. He would be a similar signing to Lambert, at Southampton, except (technically) much better – with their ability to play lone front-man and bring others into play. He would be stand-in/competition for Harry Kane and not automatic first choice. He’s played a pretty decent understudy role for Juventus (third choice, actually), who weren’t too shabby last season – were they? And he still had pretty decent ratios – meaning he can handle bench-time. Not all young footballers are flops if they haven’t made it by age 17 YO, and not all older players have ‘nothing’ to offer once they get to 29/30 YO. He is reputedly also very good as a role model/mentor to younger players.
It is up to each individual to make his/her mind up about him – but they are the arguments I would make to counter yours. There’s no guarantee that he would be a success, and you may well turn out to be right. But I believe the clincher is that he played for Bielsa at Bilbao and so pretty much knows the system and what to expect. Maybe they know what they are doing 🙂