The freezing and boiling points of water aren’t as well defined as you think. Atmospheric pressure and contaminates in the water can very easily change it.
We now base our definition of Kelvin on absolute 0 and the triple point of pure water.
Odds are that our current scale doesn’t line up exactly with the one proposed by Celsius, but even if it’s off by a whole degree, we probably won’t notice because of how imprecise thermometers have been for centuries.
The point is now we have a very good idea of how it all lines up.
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