It isn't discussing "place", it is discussing "place=island" and distinguishes this tag from others. In fact, taginfo shows only 6 hits for "place=rock" and zero for "place=rocks". That is a strong indication that the proper use of "rock" is in conjunction with "seamark". Also keep in mind that rock can also mean a landmark on the ground. To proper indicate that the rock is in the sea, you have to use "seamark".

When I searched for the tag "islet", I was referred to the overview page at: osm.wiki/DE_talk:Tag:place%3Disland
Most is in German, but in the English section the difference between rock and islet is made by size. Since the dimension of the objects was smaller than 20m at least in one direction (if not both), I opted for "rock".