The majority of dragon pokemon have a 4x weakness to ice, that's mainly why I put that sentence on the page. You are right in a way - Dialga, Palkia and Kingdra don't have many weaknesses. But Reshiram and Zekrom have more defensive strengths.
Reshiram will be weak to ground, rock and dragon... but the types fire, electric, grass, bug and steel are weak against dragon/fire.
Zekrom will be weak to ice, ground and dragon... but the types fire, water, electric, grass, flying and steel are weak against dragon/electric.
So overall they have fewer weaknesses.