That's a massive exaggeration. I used to live near Harbin, and the summers there were pretty damn hot. You'd also get about 6-7 frost-free months, not just 3. It's also not just the big cities that get really hot either - pretty much all of China excluding really mountainous places like Tibet get really hot in the summer. It's just that places like NE/NW China & Tibet get extremely cold in the winter.
Its pretty cold near the black sea parts and north eastern parts