> You cannot conclude that USA has excellent educational system if it delivers the great outcomes for certain advantaged subgroups.

To the contrary, you could totally conclude from this that US has excellent educational system at least with respect to these, as you call them, “advantaged” groups. At best you could argue that the system is not so excellent for other, less well performing groups. However, these less well performing groups still perform better in US system than matched groups outside the US, so it is indeed the case that for both higher and lower performing groups, US educational system beats all other systems around the world.