no, that’s not true. although there is a limit for how large of a file community members can load into the system for their places, iNat staff can load whatever they need to. so the boundaries for the “standard” places are as complex as they need to be in the system. (as noted above, you can see this in the greater precision in the boundaries for USA places, which come from a different source than the boundaries for most of the rest of the world.)
it’s just hard for a single source to have every boundary perfect and at a high precision for every country, state, and county equivalent in the world. and as barbetsmith noted, some boundaries just have not been defined very precisely (or have not been agreed upon), even before anyone tries to represent them in a GIS system. look at my earlier example (requoted below), and you’ll see that no source matches another source exactly for any country. even the boundary for a country like Monaco, which you would think should have the easiest of borders to define, differs between all of these sources.