But minimum flight mass is not a performance figure.
It's a fixed value, where the mass of the aircraft has to be a certain amount of lb/kg. Even if taking off from an airfield with an elevation of 5000 feet, where the weight theoretically would be slightly lower for a specified mass.
The mistake is that Diamond mentions weight for lb and mass for kg. Both would be mass, regardless of unit used.
Taken with the usual grain of salt, wikipedia explains this nicely:
"The pound or pound-mass (abbreviations: lb, lbm, lbm, ℔[1] ) is a unit of mass used in the imperial, United States customary and other systems of measurement. A number of different definitions have been used, the most common today being the international avoirdupois pound which is legally defined as exactly 0.45359237 kilograms."
"Usage of the unqualified term pound reflects the historical conflation of mass and weight resulting from the near uniformity of gravity on Earth"