He outdueled an ambidexterous, acrobatic combatant who had been trained solely for the purpose of countering/killing Plagueis. His former master also recognized him as a master duelist. He dislikes saber combat, it doesn't mean he isn't good at it (he is lacking in feats though, to be sure). You just always make it seem like he's fodder with a blade and wouldn't be able to outduel any half-competent swordsman.
Likewise, Kyp isn't a complete n00b, but he's still something like 9 in Force and 7 in Sabers.
Plagueis's melee is mostly a function of his force augmentation, where his massive force powers really kick in. I mean, everybody's claimed to be a master duelist...even guys like Obi Wan. Doesn't mean everybody is, in fact, a master duelist relative to others with that title.