How will the funds of the validator be slashed if the evidence transaction submission happens after the unbonding period is over (validator funds are spent).
You can’t slash them anymore.
The subsequent attack is for a coalition of 2/3+ old validators that are now unbonded to produce an alternative version of the chain. This is called a “Long-range attack”.
This attack will not affect synced client, but it could affect light-clients that were offline for more than the unbonding periood, or any client that syncs for the first time. This is where the concept of “weak subjectivity” comes into play. In practice, new clients and light clients that were offline for to long will not sync from a state that is older than the unbonding period.