Read the article, there are paragraphs that explain why it's called a virtue.
First, as noted above, assertion to one can be aggression to another, thus questionable high moral standards. However, if both parties agree it's not aggressive, then is it indeed morally good to be assertive? To me, morally good is a higher bar than simply being non-aggressive.
I think if virtue is defined on moral grounds, assertiveness is not consistently enough placed into that category.
Not if you understand what the two words mean.
"assertiveness" - the quality of expressing opinions or desires in a strong and confident way, so that people take notice
"aggression" - feelings of anger and hate that may result in threatening or violent behaviour.
Edit: removed 92% of snark.
Others may perceive this "assertive" behavior as angry or threatening. So, even with those definitions, Jobs for instance would be considered both assertive and aggressive.
So, labeling assertiveness as good or bad morally: IMO, it's conditional.
This is conceptually similar to the idea of addiction. Physical dependency is one component, but the definition I'm most familiar with is that it becomes an addition (instead of use/misuse) once it has a significant negative impact on your life.
YMMV, but wanted to see if I could jump in to help clear up. It's not a comment on your definition, btw - just the context of the discussion.