Not all that dumb. There are campers who are like that 1 guy you witnessed and there are real good players who go 5+ kds with 30+ kills. Of course it doesn't happen all the time but it does happen often for good players.
Just so you don't get confused, I'm talking about the ones who play solo 95% of the time. Not the ones who come in as a team and pun stomp other players.
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For sure there are people that good. But the vast majority of players with super high K/Ds are just camper or people who drop into control and don't play the objectives. Since the majority of people with high K/Ds are just campers it's not a good way to tell the good players from the bad.