There's no evidence that Michael Rae has an unusual or wasteful metabolism. 1900 kcal/day really isn't very much. With his size and input level (CI), a moderate activity level (CO) would keep him in energy balance.
Note that there is no reliable evidence that caloric restriction to the point of having an underweight BMI will actually extend human lifespans. The experiments have all been done on smaller lab animals living in safe little cages where they are protected from injuries and infectious diseases. Under nourished humans are at severe risk of sarcopenia as they age.
The Minnesota starvation study showed that the formulas fail when extreme weight loss occurs. So if someone starts with a BMI of 22-24 or so and then tries to get to 15.6, will burn way fewer calories than predicted by the calculators/formulas. 1900 is not much but a BMI of 15.6 is pretty low. He did not start that low, so I wonder how much he was eating before he embarked on this experiment. As for extending lifespan, agree, that the data is iffy on that.
Note that there is no reliable evidence that caloric restriction to the point of having an underweight BMI will actually extend human lifespans. The experiments have all been done on smaller lab animals living in safe little cages where they are protected from injuries and infectious diseases. Under nourished humans are at severe risk of sarcopenia as they age.
https://peterattiamd.com/outlive/