I think we also have to keep perspective of the environment that Ansu Fati is currently performing in. The current Barcelona is slow-tempo, methodical, built for Messi and with few off the ball movements.
Even if Ansu has grown up with the system, it takes a special talent to start and end the season so well at such a young age. A managerial change, plenty of locker room issues. He has had a great season.
So when compared to Neymar, who started in a much rawer and spacious Brazilian football (yes I know the crowds are rawkus) in a Santos team that was expansious and attack built (their Libertadores gen babied by Neymar and Ganso was seriously good).
Or Mbappe who broke out in a Monaco team which was vertical, pacy and build with all the best talents of Ligue 1 who have gone on to be key players in Premier League Champions, Champions League winners and other top teams.
Or even Mason Greenwood who is performing fantasically in a incredibly pacy and vertical Man Utd that has quite a simplistic system.
Perhaps you put Ansu in one of those other 3 setups and he would look even better. Perhaps you put one of the other three in this Barcelona side this season and they would not look as good.
BTW here is Ansu last season in the youth team where he looks like almost a different player.
As for tiers. He is clearly in the current day tier 1. How many other talents have broken through at 16/17 years old in a top 5-10 football club in the past season or two? Not many.