Thanks @sbushes but you misunderstood my request: that’s not what I am asking.
I want to find observations from their comments, not getting comments from an observation.
Following your example, Martina’s comment body says “it is probably P. formosum”.
What I want is to be able to find this observation by searching for text in comments’ bodies all across iNaturalist observations (i.e., searching for “formosum”, should return this and other observations containing that word in any comment’s body).
Similarly, searching for either “user1”, “user2”, “user4” or “whitish” should return the observation in my example above (because they all are words present in at least one comment body).
But not “user3”, because it’s not mentioned in any comments (although that user has made a comment).