Presumably users are allowed to repost [some] publicly available content, whether their own, or themes, projects, and highlights, when properly attributed?
[Apparently I misinterpreted the question. Got it backwards. What I wrote would be a correct answer to something else. Sorry.]
No. iNaturalist users are not allowed to repost content from other sources. Why? iNaturalist is building up a scientifically useful database of records of the organism interacted with by the user, in the place reported, at the time reported. The photos and audio recordings are vouchers. That means that they allow researchers to check the data, to confirm or deny the records. A photo taken by someone else at some other time & place does not allow that kind of checking, even if it is licensed so that people can use it.
I think what is being asked is whether it’s okay to repost things like Observations of the Day on other platforms, not post content from other platforms on iNaturalist.
Since every iNat record has a unique URL, you can post links to any iNat record whether yours or someone else’s on any website or send by email. I often share iNat records that way.