What was Feynman talking about?

There’s a bit in the Feynman lectures about deriving the 2nd law, quoted below.
The details seem a bit off – as far as I know the Celsius scale was determined by
dividing the range of liquid water (between freezing and boiling) into 100 evenly spaced
increments of the expansion of mercury. So a. afaict water’s expansion isn’t/wasn’t used for measurement, in fact the density of water isn’t even monotonic with temp, and b. when using celsius, the degrees are even. Possibly both can be resolved if expansion of water was originally used, then replaced by that of mercury – but I can’t find any evidence of it.

At this stage we are not going to try to find the formula for the
above increasing function of the temperature in terms of our familiar
mercury temperature scale, but instead we shall define temperature by
a new scale. At one time “the temperature” was defined arbitrarily by
dividing the expansion of water into even degrees of a certain size.
But when one then measures temperature with a mercury thermometer, one
finds that the degrees are no longer even.

So – was Feynman making this up, or raising the possibility as an example illustrating the arbitrariness of the temperature scale?