This is 100% untrue unless you're talking about menus and such. Japanese uses three other writing systems (counting Roman letters) two of which Chinese speakers are not going to know without study. A handful of the hiragana/katakana still look and are pronounced like the old Japanese kanji they came from, but those also were not Mandarin but from other dialects of middle Chinese (I want to say there was a lot of Wu, but I can't recall for certain). EDIT: and even knowing how to read them is going to be useless in the most important case, which is that they conjugate verbs, and do other very important lifting in the language.
Japanese, on the other hand, can read a lot of the kanji, but aren't going to necessarily get a lot out of anything because Chinese grammar is so radically different to Japanese.
Both will have trouble with characters whose meaning for the same character differs in the other language, and potential difficulty with characters invented in one of the countries independently.
Source: live in Japan, married to Japanese, a number of Chinese friends (mostly Taiwan and HK, but a few mainland)
Edit: sample sentence: 猫がネズミに食べされました。Here it is with the kanji for ネズミ instead, which would give Chinese readers a better chance: 猫が鼠に食べされました。However, I don't think a Chinese speaker with no internet/dictionary and no previous knowledge of Japanese is going to get the meaning correct. The second one with the kanji will fair better, but I still suspect it would be wrong.