I think that
@Chaosmancer is hoping that you will engage more forthrightly with the fact that, in classic D&D (ie OD&D somewhere up at least to the mid-80s with AD&D) it was not uncommon for the published material to contemplate, or expressly put forward, clerics of devils and demons.
And also with the fact that the 2nd ed AD&D/3E approach was not universal
even within those editions. The 3E texts put forward their approach as a default but expressly contemplated alternatives, as per the quotes from Book of Vile Darkness (p 123)and the 3E MotP (sidebar on p 8).
And here is an example from 2nd ed AD&D: The City of Greyhawk boxed set includes a number of adventure cards, including the scenario To Slay a Hierarch. The Hierarch in this adventure is a 14th level priest, with a full spell load out including the 7th level Symbol (he has the beneift of the 2nd ed AD&D spell table for clerics, which gives 7th level spells at 14th rather than 16th level), and is described as "busying himself with some last-minute proofreading of a new
Unholy Text of Asmodeus." The hierarch has a secretary who is a 9th level priest, also LE and presumably also a priest of Asmodeus.
So in fact it has never been the rule, in D&D, from 1974 at least until the end of the 4e era, that archdevils and demon lords do not and cannot have clerics.