Hmm -- it definitely strikes me as not just poorly phrased, but quite weird that they brought up racial groupings at all. Otherwise they could have just said:
If they don’t speak a tonal language at all, they fare no better on AP tests than students who grew up speaking non-tonal languages.
The racial grouping is brought up because it's a common claim that Asians (the children are half Asian if it's not clear enough from the text) have (or are born with) AP more often than other races. The author counters that while it's statistically true it might be not because of race but exposure to a tonal language (% wise more Asians speak a tonal language than Caucasians and those are the two that are relevant here).
If they don’t speak a tonal language at all, they fare no better on AP tests than students who grew up speaking non-tonal languages.