Indeed, a foreign power, having intercepted the letter from Mortimer to the address of Blake, is interested by this potential source of nuclear energy and Olrik hired to retrieve the rock. He disguises himself as a native of the country and managed to accompany Blake and Mortimer in their exploration.
It's clear that a foreign nation, having intercepted a letter from Mortimer to Blake, shows interest in this possible source of nuclear energy and Olrik is hired to retrieve the rock. He poses as a native and joins Blake and Mortimer on their expedition.