I think it's considered an "X-Book" because the original team was named "X-Force" and because almost all the members of the team are mutants
In story, some members of the team have some ties with the X-Men such as Doop with Wolverine (they've known each other for a long time) or Mr. Sensitive & Venus Dee Milo with the Professor Xavier (Guy and Dee met Xavier a long time ago and he helped them control their powers, he also created several suits for both of them for several "purposes"). Wolverine also helped the team during the original series and the original X-Force team led by Cannonball also fought against them because of the "X-Force" name.
So it's minimal but they still have some ties with the X-Men