I think the comment simply stated that when learning a language you should focus on idiomatic code and not optimizations. The latter which is probably present and justified in a standard library.

Though this argument also supports the parent comment actually: notably, standard library has much higher than usual chance and assumption of being possibly used in tight loops - with no way to relax that assumption based on actual (often closed-source) usage. So the same optimizations done in stdlib could perfectly well be considered premature if done in non-stdlib code (even if we imagined some func was not in stdlib and one needed to write exactly same func in one's code).

A library like Java Collections API sometimes uses optimizations involving bitshift etc which is too confusing for most application level code. So your comment that it's not an anti-pattern to do such things in application code is something I have to disagree with.