The answer by @PM2Ring is good, but I want to add a historical note which might shed light on what could possibly be behind that quote.
While the Celsius scale today is tied to the Kelvin scale (which is based on thermodynamics) just as @PM2Ring explains, historically that was different and defined by expansion of mercury.
So possibly, what your quote means is not the present-day definition but the original definition by Anders Celsius (modified by Linnaeus).
Celsius defined the scale in 1742, when the thermodynamic theory of temperature was not yet developed. People at the time were trying to define temperature in some meaningful way and the best they could come up with is to define it by the expansion of materials.
Celsius originally only defined two fixed points (freezing and boiling of water) and the scale in between by dividing the arm of the mercury thermometer into 100 steps of equal length (he used the word "grader" as the Swedish plural form of Latin gradus=step).
In his definition, the freezing point of water would be 100 grader and the boiling point 0 grader. Note that this is the other way round from today's definition; this looks unintuitive to us today, but, again, at that time, without thermodynamics there was no reason to believe that "cold" means "less" of something (energy), so this made as much sense as any other scale.
Jean-Pierre Christin in 1743 (and, possibly independently, Carolus Linnaeus in 1744) then inverted the scale to our familiar 0° = freezing point and 100° = boiling point, and for a long time this scale was called "centigrade" ("hundred steps"); the name Celsius scale came much later.
At that time there was a lot of debate if mercury or alcohol is better; both have practical advantages. People realised their expansion is different so mercury and alcohol thermometers with linear scales don't match up, but there was no way of knowing which was "correct". So there were a lot of different scales based on different thermometers with different gradations, like Fahrenheit, Delisle, Reaumur and others. Today theses scales are no longer used or only in some marginal situations (Reaumur is still used in cheese production and Fahrenheit only in a few countries like the Cayman Islands, the Bahamas and some land mass to the Northwest of these islands). Today these scales have also, like the Celsius scale, been tied to the thermodynamic Kelvin scale, but originally they were all defined by different thermometers.