- Does that make sense?
Yes

- Would lower octane petrol be improved by a richer mixture, or was their modern efi and lambda merely getting round it, just as a modern efi bike would do?
A1: No, richness does not alter a fuel's octane rating.
A2: If you mean "will it reduce the frequency/likelihood of detonation due to a less than optimal octane rating (all other things being equal)" then the answer is yes. Richer mixtures burn cooler (to a point). Retarding the ignition will help too. Avoiding high load/low rpm.

- Will a modern bike run better at high altitude on low octane fuel than at low altitude on the same fuel?
A: Fundametally, there is less oxygen available as the altitude increases so less fuel can be burnt per cycle. Modern bikes with lambda sensors can manage the risk of detonation (retard ignition) so the modern bike will run better at low altitude vs the slightly increased detonation-resistance that altitude gives you.

- I would have thought mixture (rich at alt) is unrelated to octane (pre-ignition).
A: Do you mean detonation (Pre-ignition is initiated by an ignition source other than the spark e.g. hot spots in the combustion chamber
). True - mixture is not related to octane. Mixture is related to denonation, especially in the context of low octane, which is what I think you mean.

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