No they aren't. I got two pairs of jeans from the same (house) brand in the same store at the same time. I tried on one of them in the store, and naively assumed the other would fit too.

There was a 5 cm (2 inch) difference.

I don't think she was implying that the sizes are always correct. But they are actual dimensions, i.e. inches. As opposed to "size 8", which is relatively meaningless.

What really pisses me off too is the size drifts to make you think you're going down pant sizes. So every so years you need to try clothes on again to get your new number.

Pair that with covid not letting you try things on and it just plain sucks. Sometimes I have to try shotgunning it so I'm not making 3 trips to the store just to try a new number.

The inseam is but the waist usually isn’t. It’s just standardized between brands much more than women’s jeans.