Depending on the “Licensing” choice of the obsever for an observation, iNaturalist is not necessarily forced to accept the deletion of the observation. Otherwise, what is “Licensing” for?
Moreover, if at any point in time the “Licensing” was public domain, anyone (including iNaturalist) who got the content at that time can forever make use of it (and redistribute it under the same terms). The same logic can be adapted to other “Licensing” terms.
The fact that an observer can change the “Licensing” of an observation does not imply an abrogation of previous licenses granted for the same observation, as far as these licenses did not specify a time limitation.