iNat data is useful for range information.
However, it is only of very limited use for estimating abundance (i.e., populations). If you are seeing scientists use iNaturalist observations to measure population, you need to take a very careful look at their methodology.
Data about populations would normally require that observers are following some kind of systematic protocol – for example, counting how many individuals they see at a certain place at a certain time over a certain number of minutes. As a general rule, they are not. They are observing whatever happens to catch their interest. They may document one individual of a species, or several, or none at all. They may visit the same site repeatedly or different sites each time. Someone else may document the same individual – their record still counts even though that individual was already documented. A user in location A may be interested in different species than a second user in location B just a few km away, thus making it appear that the first user’s favorite species is more common in location A than in location B. These are just a few examples.