[Someone]

It also never was by definition. The meter was originally defined as (https://en.wikipedia.org/wiki/History_of_the_metre) “one ten-millionth of the shortest distance from the North Pole to the equator passing through Paris, assuming an Earth's flattening of 1/334”

You have to add the fact that earth is almost spherical to go, from there, to “the length of the equator is about 40 million meters”.

Reading that Wikipedia page, I think it already was known when the meter was defined that the polar diameter of the Earth is smaller than its equatorial diameter.