Well, considering that Chinese is mostly a linguistic isolate, that doesn't necessarily contradict the thesis. Japanese has a writing system and a lot of vocabulary borrowed from Chinese, and I know less about Korean but I think it also has plenty of vocabulary borrowed from Chinese. Do any of your friends natively speak one of those languages and still consider Mandarin more difficult than, say, English?
I'm a Chinese, and I can easily understand written kanji (one of the 3 Japanese writing systems) as it's essentially traditional Chinese characters, but the pronunciation is a whole different game altogether. From this I'm assuming that it's the same for Japanese speakers to recognise some Chinese characters, thus making it easier for them to learn.
In Japan, you learn both Japanese and Chinese readings for kanji. Except the Chinese readings in China are different to the ones in Japan. A lot of people thought I was strange when I was pointing things out on a menu, but couldn't say what they were. lol
Wait so the Chinese reading in Japan is different from the Chinese reading in China and most parts of the Chinese speaking world (ignoring the influences of accents)?
方法 (method)
In Chinese: fāngfǎ "FONG-fah"
In Japanese: houhou "HOE-hoe"
日常 (everyday, ordinary)
In Chinese: rìcháng "ZI-tchong"
In Japanese: nichijou "NEE-chee-joe"
七月 (July)
In Chinese: qīyuè "CHEE-yue"
In Japanese: shichigatsu "SHEE-chee-gah-tsoo"
(Yes, 'shichigatsu' is seriously the Chinese reading of July; the Japanese reading would be 'nanatsuki'.)
And some that are similar:
開始 (start)
In Chinese: kāishǐ "KAI-tsi"
In Japanese: kaishi "kai-shee"
第三 (third)
In Chinese: dìsān "dee-SAN"
In Japanese: daisan "dai-san"
(Note that the pronunciation guide is somewhat approximate since certain sounds don't map well and English vowel pronunciation is a mess.)
I've heard that the Japanese on'yomi (Chinese) readings are generally closer to Old Chinese than modern Chinese is. Also, notice that Chinese is tonal and Japanese isn't.
