Even if these letters represented the exact same sound, Hangul would still be an alphabetic system, wouldn't it? There are plenty of combinations of glyphs which map to the same sounds in English, e.g. "ay" in "play" and "eigh" in "sleigh" or "weigh". (Although if you're making a point about only the second part of the quote — "each symbol represents separate phoneme" — then I apologize.)

I think the author was trying to distinguish between Hangul and, say, written Chinese or Japanese Kanji which are logographic. I've known lots of people in the US who assume the Korean written language functions similarly because "all Asian languages are like that".