I can't understand why people say this. Great/championship teams win some games like this, and they lose some games like this.
Think about it: most championship teams go 12-4, 13-3, occasionally better. What do you think most of those losses look like? They're not blowouts, and they're not all heavyweight bouts with equals. They're mostly ugly close games where they let a lesser team stick around and steal one away.
Great teams aren't great because they win a bunch of games like this. They're great because they destroy lesser teams, like we did the Browns and Bears, because they never get blown out, and because they don't lose more than their share of the inevitable ugly games that come along.
If Thielen didn't fumble and the Vikings eked out a win over us, it wouldn't say anything different about the team: we'd be 10-2 and our path to HFA would be a bit harder, but we wouldn't learn anything about the Cowboys than we learned from the close ugly win.