Sentence Correction

This is in reference to OG 2017 question #732

Laos has a land area comparable to that of Great Britain but a population of only four million, many of whom are members of hill tribes ensconced in the virtually inaccessible mountain valleys of the north.

The underlined portion is the correct answer. I disregarded this answer because I thought it was too vague as to whether the population of only four million was in reference to Laos or to Great Britain. Why is this acceptable? Thanks.