It's weak evidence for the first, proposition, but I don't think it's any evidence for the second. I suspect you're right, but to play devils advocate, it could be that training/selecting for grit could be more important for future success than the learning is.
It's at least a little evidence that training people to endure boredom is a bad idea, since boredom is part of inefficient learning.
School has enough prestige that I think a lot of people come out of it believing that if they can't learn in a school like environment, they can't learn at all. Sometimes they just apply it to one subject, sometimes to all subjects commonly taught in school, sometimes they give up on learning.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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