Not all blades were constructed in the same way and of the same materials, but the Chinese are noted for originating binary swords.
Ancient Chinese metallurgy recorded six different bronzes. In practice, the archeological record shows a much wider array of proportions, if only because much bronze was likely to have been recycled, but it seems clear that they would have known about the properties of high-tin and low-tin alloys. High-tin bronze is very hard, as you note, and very resistant to corrosion. It thus would not deform in battle (i.e. bend permanently instead of springing back into shape), and would hold its edge for a long time, at the expense of brittleness.
Early high-tin blades in particular tended to be short (50-60cm) on account of the brittleness, but by the late "Spring and Autumn Period" (~500 BC), the Chinese were quite advanced in metalworking, and were casting bimetallic swords. The core of the sword was softer and more resilient low-tin bronze, whereas the edges were formed from harder high-tin bronze.
(from http://thomaschen.freewebspace.com/photo3.html , original source not provided)
One should note also that swords in this period were a weapon of the nobility. Tin was rare in the south, and would have been expensive. This technique was also too complex and expensive for mass production. The primary weapon for foot soldiers of this era would have been the dagger-axes, and later spears and halberds, and it was improvements in ironworking and steel making (originating in the south) in the Han Dynasty that would displace the bronze sword.