Heart rates must be Lognormal in distribution. Simply, it is not possible to have a negative heart rate and at low variance (and a mean > 6 standard deviations away from 0), the lognormal behaves like a normal.
Incidentally I failed to understand from San-Millan’s paper(s) the 2 mmol lactate threshold claimed in the podcast with Petter Attia and elsewhere. I don’t see a threshold. Even for athletes (top graph below) there is a mix outside asymptote (Lactate >5 mmol becomes 0 fat oxidation).
San-Millán, I. and Brooks, G.A., 2018. Assessment of metabolic flexibility by means of measuring blood lactate, fat, and carbohydrate oxidation responses to exercise in professional endurance athletes and less-fit individuals. Sports medicine, 48(2), pp.467-479.