The only 2 ways to "hack" a Mark are Brute Force (Cybercombat + Logic [Attack] ) or Hack on the Fly (Hacking + Logic [Sleaze] )
So technically you are correct. You don't need Cybercombat, if you have Hacking. But the point Haywire was going for, is that if you have the Cracking Group of skills, you are what the book would call a hacker. Maybe not a good one, but you are one still. If you don't want to make a hacker, then your best option is to buy a Deck, and install an Agent program on it. It's like a baby AI that only knows hacking. It's got the Computer, Hacking, and Cybercombat skills. You just plug it in, or point it in the direction of the target, and it does all the work. It's not a replacement for a team Decker though.