Reliably means close enough to be useful, but the estimation is also quite precise where most populations live.

My use case which led me to discover this fact is sort of documented here: https://twitter.com/mholt6/status/1695685022710477043 -- even if my use case did have whole kilometers of displacement, it wouldn't likely be near the poles, and if it was, the answer would be, "Meh, we get it, you're at the pole."

The Earth diameter at the equator is 43 km larger than at the poles.

It's like the Earth orbit: we learn in school that it's an ellipse, but we are never actually given a sense of the shape, and most of the drawings give a completely wrong idea.

The title of the post could make you think that a degree of latitude varies by less than a decimeter, but that is of course not true. But close enough for many practical purposes, definitely.