People forget that in Club World Cup not all top teams participated e.g. Liverpool, Arsenal, Barcelona, and Napoli. If they participated, Chelsea might already got beaten by one of the team I mentioned before.
There's a difference between a league and short competition, a long league even the team beat top teams, they still not able to become a champion because it's about consistency. We have seen ugly lose where small teams beat big teams, this also a problem.
Napoli does not look to me like a team capable of stopping Chelsea from winning the club world cup actually, they would have done the much they can but they are not as tough as you are wanting to make them appear except in their domestic league of which beyond they are not that tough enough. Liverpool, Arsenal and Barcelona would have actually pose a serious threat to Chelsea but I guess it was Chelsea's lucky year so we give it to then.