Earlier quoted context omitted.
Because when you learn Japanese, you have to learn kanji, the more traditional strokes (non-simplified), both Japanese and Chinese sounds, and the grammar is closer to Korean (and closer pronunciation for a lot of words). So, it's better to learn Japanese, quickly pick up Korean, then learn Chinese. Chinese last since you have to learn simplified characters, the grammar is different, tonality is quite different, etc.
Doesn't the order matter only if it's harder to do this the other way around? Anecdotally for me, learning Chinese first with simplified characters was totally fine, and without formal study, I can read traditional characters fine 90% of the time and guess a remaining ~8% of characters I don't immediately recognize for a total ~98% comprehension, kind of like figuring out how to read medieval gothic font. Knowing Chi…
AIUI that's not a recommended approach; my friend is learning Chinese and his teacher was insistent that he should learn the traditional characters first because it's much easier to pick up the simplified characters from the traditional ones than the other way around.