Nothing in music is illegal, you won't go to jail!
A skip is any movement that isn't step-wise motion of whole or half steps.
It's just triadic arpeggiation in the passage you pointed out. Something that works musically, but isn't the point of the Gradus, so Fux glossed over it.
Don't miss the forest for the trees. The Gradus isn't 100% consistent or meant to explain everything. Follow his instructions as closely as possible, and understand that the occasional deviations like this (and those in m.2, 5, and 6) illustrate some artistic license is OK provided you try to diligently follow the point of each species.
Are far as the artistic license thing goes, keep in mind there is often a trade off between options, like concern for line over harmony or vice versa, and it can be good to examine the possibilities. In m. 9 if the c.p. had resolved the skip by a step in the opposite direction, it would have landed on a tritone on a strong beat. We don't want that dissonance so we do something else despite the general rule to resolve skips with a step in the opposite direction. Also, m.9 is about harmonizing the F of the c.f. Notice how Fux handles all the bars with F in the c.f. All of them arpeggiate the minor tonic chord. I know we aren't supposed to think of species counterpoint in harmonic/chord terms, but it's hard to not admit what's going on in those bars.
It's wrong to read the Gradus as rules and laws of music. The aim is to follow the graded species and develop a disciplined craft along with an appreciation for the overall aesthetics of counterpoint. In other words it is not a complete composition manual. It's more of a disciplined training program for the essentials of counterpoint.