Even a kid who went to a yeshiva that managed to avoid most secular studies isn’t all that worse off if he wants to enter a secular field. In terms of language, he can read four languages (Hebrew, Aramaic, Yiddish and English, from different families (two Aryan, two Semitic), which is a big asset in acquiring any language based skills. He has experience studying ancient texts, including and especially legal ones, in the original language with medieval commentaries in the original langauge (skills usually attempted in public schools only in the graduate school level). He is used to working independently. While he will have to do some work in some subjects, he’s starting with a humanities background that is quite rare today (though was common among the secular elite a few centuries ago).
Note that in the public school,s academics is hardly a full time focus (cf sports, clubs, social activities, etc.). Most subjects are serious stripped down and oversimplified (imagine studying science or the ‘hard” social sciences without having learned calculus and statistics, which are rarely taught before 12th grade – or trying to study foreign language and literature without knowing a foreign language).
A yeshiva student who was a success at learning Torah and decides to pursue a job that require secular advance education probably can manage to a GED in a few months, and some AP or CLEPs within a few years, and can then easily get into a college. Many students follow this route in the real world (e.g. home schoolers). If someone with an exotic background (yeshiva is sufficiently “weird” to qualify) shows up with a year or two of college credits from self-study, they can get into a good university.
I suspect the OTD Hasidim who complain they are stupid were just as bad off when they were frum, and rather than admit their own shortcomings are trying to blame their parents and teachers.