Luck does play its part, but usually on the negative side mor than the positive
Since we are talking about relative terms, a team's bad luck is another teams good luck, hence my comment. I sincerely believe we are discussing semantics here. Lets move on.
This is called "pressure". Any international rugby player will tell you that the level of the game is a step up and everything happens faster and under more pressure. There are many Super Rugby/Top14/Premiership players who were really good at domestic level but simply were not good enough to make the grade at the next level. In NZ I can think of a few...Isaac Ross, Mark Ranby, Casey Laulala & Craig Newby. Perhaps Jantjes is one of those who simply isn't good enough to handle the pressure and speed of the International game
There might be a bit of that, in many cases actually a lot, but you cannot always blame pressure and in many cases the sample size is too small to arrive to any sort of definitive conclusion. Some people crack under pressure and some people just have a bad day, week, month. There is quite a lot of statistical analysis and the results are mixed at best. You need more evidence before you conclude someone choked.
When we talk about things like these i can't help thinking about Quade Cooper. With just a tiny bit of cherry picking i can easily put a couple of games together to show how the guy plays better under pressure in a big stage or pick another couple and show how the guy is amazing at unimportant games and drops the ball when it matters and the stakes are high.
Personally, i think the guy is just erratic. You never know which one is going to show up, regardless of the pressure. My opinion course, you don't have to agree, but i hope you see the point i am trying to make.
You are not going to like this example but bear with me please: following your logic, that when a better team loses against a worse one (relative terms again) you can explain such event by "pressure", you
could easily argue that the ABs WCs loses against Australia in 2003, France in 2007 were because of the ABs not being able to handle the pressure.
I do not believe that for a second, and i assume you do not either. What i do believe, just to avoid any potential misunderstandings here, is that the best team had a bad day and the worst team had a good one, and that was enough to compensate for the difference in skill, preparation, training, coaching and talent. It happens. Sucks when it happens at a world cup but sometimes it is impossible to predict how a ball is going to bounce, regardless on whether it is friendly game at the local club or the world cup final.
Cheers