That's a comparison between the most talented people of South Korea where learning English language is legally obligatory - it isn't in the US - working on official documents sent to an important neighboring nation-state about an important global event and relatively common people posting on not-so-important places. And it's their job... to make the grammar and spelling perfect. I usually turn on the automatic grammar and spelling and style check in Microsoft Office applications and Google Chrome web browser to reduce error rates - the resuts are first sent to proofreaders anyway.
That document is a short letter, but it happened on a day the president visited the island... and he also said the Japanese emperor must apologize in public before thinking of visiting South Korea. That's why the letter's posted everywhere - in South Korea and Japan.
Limiting national languages to one per each nation-state is not the same thing as having one common language to share among all nations and all states. Globalization is always an enemy of nationalization. English as THE standard world language limits the power of French in France, Korean in South Korea, Chinese in the Greater China, but it makes the peoples of a little over 50 African countries have something to help them communicate with four billion individuals living in Asia. It is much better than having none.
Japanese, South Koreans, and Chinese in both mainland China and Taiwan are not good at writing in English not because they think English is not important to them but because their national efforts to learn English failed totally. They say it is not important only AFTER they learned they had failed. That forces them to become even more dependent on making their politics and economic policies more nationalized which in turn makes the learning and education more disasterous. So they had to invent theories and histories to make their language and heritage look better, but each president and ministers pay more attention and lip services on public English education and many parents spend something like US$1,000 per child per month on English. Being able to speak English mixed with American slangs, especially the kind of slangs employed by the least-educated teenagers, even make them look sexier.
I don't care much about South Koreans not being able to write and read because the Korean language including Hangul and Korean-Chinese characters is least compatible with almost all European languages. For native speakers of the English language, especially the well-educated professors who have taught English to some of the best college students of South Korea in Seoul for at least several years, it is not so difficult to understand English spoken and written by their students. For the average Chinese and South Koreans, it is easier to understand English spoken by native speakers of the language in Britain and North America than for them to understand English spoken and written by peoples of Africa and Indian subcontinent. Many parts of the Middle East and southeastern Asia were British colonies, but their English cannot be easily understood by Latin Americans. In a future world, those peoples of nearly 200 countries whose most important official language is not English are going to be networked with billions of highly mobile devices. Air travel and emmigration are too costly solutions for most of them, but technologies are evolving in a way to make physical contacts more expensive and symbolic contacts less expensive so that most of the very meaningful social changes on the global scale are going to be generated through mobile communication.