Back on topic, heres what the word "retro" actually stands for:
retro-
prefix meaning "backwards, behind," from L. retro (prep.) "backward, back, behind," probably originally the ablative form of *reteros, based on re- "back." Common in combinations in post-classical Latin.
This is relevant for the word retrocomputing.
When someone builds (or buys, whatever) a 3DFX rig for playing glide games, his goal is a computer for reliving the past. No arguments there.
Since this is a computer which use has the same meaning in the sense that it's about going "backwards" as a C64, both a C64 and a 3DFX rig are technically retrocomputers.
And in the same way a 3DFX rig is technically a retrocomputer, playing glide games on it's original hardware is technically retrogaming.
The exclusion of these are thus technically wrong.
Why would anyone (ok, here I go bitching again since I'm creating a lil fanclub of my own hehe 😁 )consider a C64 as retro, but a 3DFX rig not?
Possibly because of personal reasons, since technically he has no reason to make a distinction between the 2 when it comes to the words retrogaming and retrocomputing. Retro is about "going backwards", it's where the word originated from.
The word "vintage" can be similarly interpreted. Calling a Pentium 3 running 98SE and a glide graphics card is technically a vintage computer. You may disagree, but technically it's actually the truth!
I was quite surprised actually. The word "vintage" doesn't even exist in Dutch so this was kinda a revelation of small sort 😜