 he means that it is the surface of the earth. so whoever is high and whoever is at a level are the same. and whoever is hidden from it is like someone who appears in the desert. he means that no one was safe from its rain. this is like what alhudhali said
يريد أنه طبق الارض. فمن كان فى الارتفاع، ومن كان فى الاستواء سواء. ومن استكن منه فهو كمن ظهر فى الصحراء. يريد أنه لم يسلم من مطره أحد. وهذا مثل قول الهذلى