The reason to assume that Dothraki climate resembles that of Earth steppe peoples is, as quoted in the essay, because Martin claimed to have based the Dothraki

The reason to assume that Dothraki climate resembles that of Earth steppe peoples is, as quoted in the essay, because Martin claimed to have based the Dothraki on them. Those Steppe peoples have adapted their clothing and cultures to the climate they live within. If the Dothraki are supposed to resemble these people culturally then their clothing should reflect the climate of Earth's steppe peoples.