Several accounts of how he originally defined his scale exist, but the original paper suggests the lower defining point, 0 °F, was established as the freezing temperature of a solution of brine made from a mixture of water, ice, and ammonium chloride (a salt).[2][3] The other limit established was his best estimate of the average human body temperature, originally set at 90 °F, then 96 °F (about 2.6 °F less than the modern value due to a later redefinition of the scale).[2]

Yeah, the freezing point of a random solution, and the perceived temperature of the human body makes as much sense as the freezing and boiling point of water for reference. Kelvin is objectively better, but centigrade is immediately understandable as it uses easily understandable reference points.