People are talking about Son Heung-min being S-tier below

What matters is what the criteria for S-tier is.
Personally, I consider S-tier to be players just below the "god tier"—players like Mbappé, Neymar, Suárez, and Lewandowski.
I view Son Heung-min as an A-tier player, which is the tier directly below those players.
I think the S-tier should be reserved for those ultra-elite world-class players who are actually contenders for the Ballon d'Or.