>> timestamp is coming from the operating system where the client library is running.

The client can set the timestamp to any value of its choice. It does not have to correspond to clock time.

>> so unless all your write request are issued by the same machine then you are guaranteed to face the problem where the client machine clocks are not in sync.

It's not about all writes. It's about ensuring that all writes to a single partition (specifically, a single column within a single partition) are done using a source of monotonic integers to ensure ordering.