To bloviate a bit more as any good wordwind would:

Well, I've been won over. The more I think about the metaphor, the more I believe it works. There's coincidence and there's coincidence: There's coincidence that is surprising--even startling in its aura of pretermined inevitability (Jungian synchronicity)--and there's coincidence that we say "Pish-posh!" to. It seems to me that this grammarian was checking out pedigree of some usage, and reached a point at which that search evaporated into that area of coincidence that was misty at best--the "pish-posh" sort of coincidence that lacks stability and a pause-for-thought about inevitability.