HCL won't rehire their Ex-employees?

They do rehire as per policy

As per policy you are eligible for re-rehire provided you have served your notice period. Now here is where it gets a bit tricky, if within a span of 2 years you are returning, you will be hired on same band and salary on which you left. Post 2 years, things might change.

If u have served the notice period and have not absconded, I suppose u r eligible for rehire.

It depends on requirement no connection to policy, policy can be change as per requirements

I don't have specific details on HCL's rehiring policies. Companies often have different policies, so it's best to check directly with HCL for accurate information.