You might find this related discussion thread informative:
https://forum.inaturalist.org/t/add-all-species-from-flora-of-china/5171
In short, names are added to the iNat database as needed. It is not the objective of the site to have a complete taxonomic database, only to the extent needed to support observations and IDs. I can’t speak to a policy (if any) about building checklists that extend beyond actual observations or IDs.
From the Curator Guide, the site’s current taxonomic framework for vascular plants is:
Vascular plants (Tracheophyta) : Kew’s Plants of the World Online (POWO) with deviations described here. As of August 2018, current policy excludes following POWO for ferns and genera that start with “P”. See this journal post for more information.