I am quite sure this is not an average case. He likely visited a high school olympiad team. Tbh, CS in Vietnam educational system is still treated like "third class citizen."
(I am Vietnamese, went to a gifted high school and had exposure with competitive programming)
He most likely visited an Olympiad team, where they solve problems like these to compete. Most other schools, such as the ones I went to, had IT/CS as a side class (meaning they don't get that much attention from both students and teachers alike) rather than a main class (think Math, English, Vietnamese etc).
Some pictures from the school: https://imgur.com/a/yaeGDCo
I wonder why Linux is not becoming more widespread in such places. Something like Xubuntu runs well on mediocre hardware and is completely free. Compared to such an old Windows like XP it also wins in terms of usability and software availability, not to mention security.
My guess is that it's simply a lack of awareness that this FOSS ecosystem exists.