Agreed.
I suppose I was referring to different positions within an individual discipline. If we're talking MMA, that's still a combat sport albeit one closer to real fighting than boxing or wrestling. But we've seen that at the highest level, top MMA fighters almost always came up first through an individual discipline, whether wrestling, BJJ or striking, and attained at least full proficiency (if not elite level) before branching into MMA.
But in say BJJ, if you ONLY know how to play TD into top game with no understanding of guard retention or recomposing, not only would you be at a big disadvantage vs. other pure BJJ players, it would be a stretch to even call you a BJJ practitioner. You need at least a rudimentary understanding of every position in BJJ to be a proficient BJJer. If you chose to then make that your specialization base to pursue MMA, you would need to further round it out with striking, wrestling control, cage control, GnP, etc. to be proficient under the new rule set.
Even Conor McGregor has pretty good grappling just like Khabib has pretty good striking. Neither are elite at their B games but if they had NO proficiency there, neither would have been as successful.