Thanks for the correction and good words, yes, two coordinates! I also just stumbled into a bounding circle today, the syntax being slightly different. I do not know how I missed this, but a single coordinate with a radius generates a circle. Perhaps not as useful for a yard list, unless one happens to have a circular yard, but this option actually works well for me.
https://www.inaturalist.org/observations?lat=6.9542&lng=158.20395&radius=0.03
The radius is in kilometers. I expect that setting up a place for a project would make the place and perhaps the project “discoverable” which may carry privacy implications as noted above.