We must be specific about what the claim is. It's not sufficient to wave our hands and say something vague like the test "works well" in those circumstances -- that is not what was examined in order to make the statement.
Both statements are specifically about accuracy of the significance level (a.k.a. "level-robustness").
That is to say, the type I error rate is claimed not to be too far from what you would calculate/choose under the (violated) assumption in those circumstances.
Even in that restricted sense, these sorts of general claims are too vague to be useful in practice, however. For example, you don't really know how large is sufficiently large for your purposes in the first case, because you don't know the population distribution (if you did, you wouldn't need to consider this issue at all!).
Of course, significance level is not the only consideration with tests. Certainly I'd hope that people care about power. Sadly, however, the direct evidence that the people who repeat these statements care much in practice is weak when common statements like these so rarely are accompanied by the merest mention of what happens with power.
In the first case, large samples don't save you when you're looking at relative efficiency (the relative sample sizes needed to achieve a given level of power) -- and relative efficiency can be arbitrarily poor in large samples -- so if your sample sizes were large because your anticipated effect size was small, you might have some potentially serious issues.