Not just the walkthrough (that was just an easily accessible visual example), all DPCs communications, from the pre-release thread to now, even in his polls etc. He's made a hard distinction between main girls and side girls throughout development.
E8 wasn't the first time there were material differences in game between main & side girls. The MGs are still the only girls who have RP points for instance. Quinn & all the other SGs miss out, presumably because there's no planned 'relationship' with them. Perhaps more importantly, the MC is heavily involved with the MGs personal lives, regardless of whether he's pursuing them romantically or not. While it's possible for the MC to have next to nothing to do with Quinn & the other SGs should you choose. The difference has seemed pretty stark to me throughout the game.
Whilst you're correct in defining the technical distinction between MGs and SGs (the former being involved with MC in the main story regardless of romantic choices, the latter not), I don't agree with your conclusions about what this means for relationships and endings with the various girls, particularly for Quinn.
It seems to me that apart from in the guide and polls, far from maintaining a "hard distinction", DPC has deliberately and repeatedly blurred the line between the two categories when it comes to the game/story content itself. Quinn, for example, has had her own prologue, her own hangout, her own path, and her own animation at the end of episode 8. These are all features she has in common with MGs, not SGs, and yet a strict interpretation of the official guide would have us believe she is categorically more similar to Cathy, someone we've barely seen for two episodes. In fact, as a result of all this, it's not uncommon for new players to say they assumed Quinn was a Main Girl, based on their experience of the game. This happens so frequently that at a certain point I think you have to accept those players are responding to something real in the game, in terms of the way Quinn's character has been purposefully developed.
It's reasonable to ask why DPC has done things in this way. For me, it's because he's interested in developing her as a romantic option with a distinct path. As I've argued before, the biggest hint about DPC's intentions in this regard is the animation at the end of episode 8. He didn't have to do that, he could've just as easily merged Quinn's route completely into the Others path, but for some reason he chose to single Quinn out, emphasising her as a love interest, not an antagonist. That animation looks back to a moment in episode 8, but I think it is also a statement of intent for how she's likely to be used as a character in future episodes, if on her route.
Just a quick word on your reference to RP: I think I'm right in saying that DPC has spoken about moving away from this as being a big determiner of relationships. There are a few examples of where it matters, in very minor ways, in the first 8 episodes, but mostly it doesn't. So I don't think we can be confident that lack of RP means no ending or relationship with a SG.
The MG/SG distinction is a real thing, as you noted, and it has an impact in the game. Bella will always cut the MC's hair and Jill will always try to protect MC by dating Tybalt, regardless of whether the MC is dating either girl. Quinn doesn't work in the same way; so far she's largely an antagonist in the main story and only involved with the MC if he's on her path. But in my view, this is a comment about the way the main story is structured, it doesn't have anything to say about the viability of the romantic options. Quinn is not an MG (so far), but she is definitely a LI. I think DPC has made this pretty clear. I don't think she's therefore limited in where her route can go just because she doesn't have the MG label.
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