Not necessarily. In fact, their definition leaves no mention of CC whatsoever.
Juggernauts are melee titans who relentlessly march down the opposition and devastate those foolish enough to get within their grasp. They are the only subclass who excel at both dealing and taking significant amounts of damage, but in turn they have a tough time closing in on targets due to their low range and extremely limited mobility.
Ofcourse, several aspects are often broken on a champion to champion basis to create unique champions. For instance, a Juggernaut with an initiation and AoE CC that's far from as reliable as a true Vanguard.
Reliability is the key factor that determines why most Vanguards are picked in the first place. Sion isn't picked as a tank for his reliable initiations but rather his pure undying tankiness with the potential of big AoE CC.
He disrupts teamfights just like any other Juggernaut does though, not through landing hard CC but rather through sheer persisting threat. He coerces enemy movement away from his team by setting up Decimating Smash in ways that persuade enemies away from them. He doesn't even have to land it to successfully fulfill this role.
Aatrox performs in a very similar manner. It's not so much that he's landing all his sweetspot Q's and CC'ing the enemy due to them, he's simply creating areas enemies want to avoid that coerce them into favorable positions for his team (while also landing key W's and Q's here and there). Same could be said for Urgot (or anyone else) who uses Righteous Glory to get in before bullying people around with sheer threat presence, which is basically what Sion does as well.
All-in--all I believe Sion to be a hybrid of sorts due to his above average tankiness and his higher-than-average potential of big CC but with a far greater lean towards Juggernaut due to how he executes it (alongside AD itemization being an option for a harder hitting Sion).