Checkmated India from doing what ? What did the Chinese think India was going to do ?
India split Pakistan in Two, created BD and still China did nothing. US tried to do something, not china.
China supply lines via India ocean is unsafe and remains vulnerable. Has Chinese diplomacy made it any safer ?
Instead China was forced to invest money in an unstable pakistan and develop gawdar port and is spending billions in developing road and rail lines in pakistan to transport goods to china via pak.
If India was a friend to china, India would have done that for free.

....... Now consider if this was a diplomatic victory or defeat.
China is forced to spend BILLIONS of $ building artificial sand castles in the SCS, just so that it can stake a claim that no one in the world recognizes. Worse, it ensure that the rest of the nations gang up against China and provide more leverage to the US. Who's diplomatic victory is that ? US or China ?
China provides Pakistan with Nukes and pakistan sends terrorist into China and stir trouble in Xinjiang.
Its only diplomatic success was establishing a listening post in coco island.
Russia provides nuclear submarine technology to India but denies them to China. Another diplomatic victory ? Especially in a time when Russia is isolated globally ?
North Korea and Kim Jong-II now hates china more than any other nation. Where is the diplomacy there ?
Mongolia hates China. The Japanese hate china. The Vietnamese hate china.
China's success is due to its willingness to do trade with the US on US terms. Pegging the yuan to the USD, investing in US Federal securities, and becoming the factory of the world by exploiting the labour and at the cost of environmental degradation. Though the returns for China has been much greater than the returns for the US. But do you consider that a diplomatic victory ? US chose china to put pressure on Russia.