This isn't totally true. Mass and volume measurements were indeed derived from the meter. A gram is a cubic centimeter of water. A liter is 1/1000 of a cubic meter. Apparently Celsius is derived from Kelvin (really just translated so 0 is the freezing point of water), which is derived using metric units in a formula that is a bit beyond me but available here:
https://www.bipm.org/en/si-base-units/kelvin
Anyway, TIL.