I'm not precisely baffled as to why there are people who think that this manga is a masterpiece.
After all, there are over a billion people worldwide who think that a clusterfk like the Qur'an is the word of god, and that its excellency is proof of that premise, although it is riddled by incomplete sentences, spelling errors, grammatical errors, practically unreadable in its original form because it lacks vowels (early Aramaic script doesn't have the vowels, they only start to appear in the 8th century AD)
and blatantly copies apocryphal Jewish and Christian gnostic sect accounts that were available in Aramaic during the time the Qur'an was created (late 7th/early 8th century till the 9th/10th century AD).
So the question is: How do you define a "masterpiece"?
If you simply put up a bad work as a standard (like the Qur'an) and subjectively measure how much more you enjoyed to read this work compared to your standard, I guess this manga can be called a "masterpiece",too.
If you objectively analyze the text without any standard in mind and just look at how well-developed certain aspects of the manga are (setting,story,characters,development,narration,design,artwork,etc.) however, it will be impossible to rate this as a masterpiece, because of the shortcomings it has in several of these categories.