You are confusing etymology with history
The etymology of drider is clear.
It was first used in Q1, published in 1980, although no doubt there were manuscripts and proofs before that that we don't have access to and may no longer exist. It was almost certainly used verbally in the TSR offices as well. Unlike most neologisms, where the definition has to be inferred, we are explicitly given one on page 26:
The drider is a special creation of Lolth's magic. When Drow of promising ability reach 6th level or slightly higher, Lolth will summon them and put them through a specially-devised test. The Drow that fail become driders.
A drider appears to be a cross between a giant spider and a Drow.
So does the word. A lexicologist, given this evidence, would have no hesitation concluding that drider is a portmanteau of drow and spider. Gary's 2007 comments don't change that and, even if that had been his intention in 1980, drow spider still fits the concept.
Historians rank sources.
We have a contemporaneous primary source document from 1980. We have what is effectively an oral history from 27 years later; also a primary source.
It is possible, possibly even likely, that the origin story of the drider was discussed in the TSR offices. All we can say with certainty is that Gary's position didn't win. It is also possible that Gary is remembering things that didn't happen - that's how memory works.