Or sometimes you get drift, and then you don't remember a smell, and this happens all the time - which is why if we re-smell something, we don't necessarily hit the same neurons, but we also remember old smells, because those didn't drift. Or is that excluded by the experiment?

My understanding was that these experiments didn't completely rule out the possibility of a stable sub-population. Also, the olfactory bulb projects to other areas besides piriform, and those might support long-term stability. My extremely speculative unfounded conjecture is that piriform is actually a novelty encoder, and plays a role in encoding new memories in a changing environment. But, I think the data don't quite support this at this time.