Superscript open circle notation in formula
According to The Manual of Scientific Style: A Guide for Authors, Editors, and Researchers 9.2.5.12


  Indicate a free radical by placing either a centered dot or a superscript dot, signifying the unshared electron, next to the chemical symbol or compound formula


This convention is also followed by the ACS. 

So it would be better if the article used the solid symbol throughout, but the solid and open symbols mean the same thing in the article.