This is a bit of a trope in places like /r/ErgoMechKeyboards/. However, it's a bit of an empty statement without further data. Of course hand sizes/shapes differ, but the question is if they differ enough to preclude designs that work for a majority of the population. If you look at e.g. hand length/width datasets, within a group (e.g. men) these are close to a normal distribution. And the spread is not that large. e.g. the different in hand width in the ANSUR II 2012 dataset is 0.6cm between the 25th and the 75th percentile for men. The difference in hand length is 1.3 cm between the 25th and 75th percentile.
So, it's very likely to be possible to design a keyboard that fits most people well enough. Sure, if your hand size is an outlier you may have to get creative.
Besides that, there is also some latitude in designing ergo keyboards for multiple hand sizes. See eg. how Moergo iterated for several years so come up with a design that works for many different hand sizes [1].
[1] https://www.moergo.com/pages/glove80-ergonomic-keyboard-desi...