First, Hashem is more than a being that has no end—He has no beginning. He is a primordial being, or a kadmon—a being that never began to exist, but always existed.
Rabbi Moshe Cordovero taught: “Every primordial being is eternal, but not every eternal being is primordial.” Normally, a non-kadmon, i.e., a being that begins at a certain point, must come to an end. However, an entity can break this rule and exist eternally without being a kadmon, for it can be eternal by divine will. Its eternality is then not inherent, but a product of divine “intervention.”
However, if something is a kadmon then it is necessarily also eternal, for something that always existed is not subject to change, and cannot therefore cease to exist at a certain point.
Since eternality does not necessarily indicate primordiality, while primordiality does necessarily include eternality, it is a much greater praise to say that a being has no beginning than to say that it has no end. This is the first reason that it is incorrect to that the “Ein Sof” in “Ohr Ein Sof” refers to Hashem’s Essence, for Hashem’s Essence is not just unending, it is unbeginning.
Second, describing something as unlimited implies that it is able to extend and reveal itself, albeit infinitely. However, Hashem’s Essence transcends all aspects of extension and revelation, and thus it is incorrect to refer to His Essence as Ein Sof.
Rather, the expression “Ohr Ein Sof” means that the ohr itself is Ein Sof.
Adapted from Sefer HaMa’amarim 5664, p. 30 ff.