I take a rather simplistic view of all this. I know, for an absolute fact, as a rare Ultimate Truth, that if I never take another puff on a cig, I will never relapse back into abusively smoking. I cannot say that with any certainty if smoke another cig.I think what you are noting is reason to be cautious about having another smoke.
I would note that I have the cold turkey / relapse experiences you speak of. I literally find it impossible to relapse fully to smoking abusively with the existence of vaping. I think it would be plausible if one were to have a smoke to satisfy some recurring craving they feel they are having AND then make impulse decision to stop vaping for awhile. Without the latter, I would argue intellectually that it would be impossible, or at least very unlikely.
The one thing that does pop up when you have the smoke, which is subtle, but I think (ex)smokers can relate to, is the idea that smoking is perhaps better than vaping. For the vaper who's been vaping for years (and not weeks or a few months), such thinking is ludicrous. Even I as dual user feel vaping is far better. But I think some can remember that when you made the transition, you possibly had thoughts of how is this vaping stuff going to equal the great satisfaction that I strongly believe I get from smoking. Some vapers speak as if all it took was one puff from an eCig to, in that moment, realize they would never smoke again, after enjoying smoking for decades. To them, I don't think this paragraph would ring true or make sense. But how many smokers transitioning do you know, or hear about, that want a flavor that tastes exactly like their brand of smokes? Or the smoker who is stubbornly transitioning and simply can't be open to the idea that one puff is going to magically transform them into vaping fanboy.
With my (open) mind, I could see it as possible that a never smoker who vapes, and then for whatever reason tries a smoke, would find smoking to be better than vaping. I think such data will at some point exist. But, I think it will be so low of a population that it'll be very easy to dismiss as an extreme rarity. I also think that if this same person found themselves fully addicted to smoking, and wanted to break that addiction, they could do so, easily, via vaping. They may like vaping less, but if they hate the addiction to smoking that developed within them, they might like vaping more than that.
I also have a lot of historical evidence that I cannot control my smoking. I could never keep my quantity smoked vs time graph even. And if that chart rises, even slightly, then inevitably I'll be back to smoking my old 2-3 PAD.
I respect that that is a personal experience. I have no issue with people that can have an occasional smoke without it relapsing into a regular habit. But I do caution new quitters about my own experiences, and to search their souls to see where they fall on that spectrum.