Each CIR index represents the time interval?

Does CIR index is 1.001602564ns (1/(499.2 MHz*2)) ?
the CIR index for the first path will be around 750. Is there a 1 ns difference between index 750 and 751?
Each index represents a distance difference of 30 cm?

On page 182 of《DW1000 User Manual 》—— Sub-Register 0x2E:1804 – LDE_RXANTD

The units here are the same as those used for system time
and time stamps, i.e. 499.2 MHz × 128, so the least significant bit about 15.65 picoseconds.

I’m a little confused about what 499.2 MHz × 128 means, Why does 499.2 MHz multiply 128?

The internal clock runs at 499.2 MHz. For some of the logic this is multiplied up by a factor of 128 giving a clock speed of 63.8976 GHz or a period of around 15.65 ps.
But if you start writing 63897.6 MHz in the documentation it’s less readable than 499.2 * 128 and also the relationship between that and the main internal clock is less obvious. It’s also far less likely that someone will enter 499.2 * 128 incorrectly in their calculations than 63897.6.

Similarly the CIR buffer is at a rate of 499.2 MHz * 2 or just under 1 GHz.

So to answer your initial question, yes each index in the CIR represents around 30 cm.

The leading edge detection will however give a results to a sub sample resolution which is why you can get better than 30 cm accuracy on ranges.