How many hosts have you got in your environment that were configured in the cluster (and then set to use a Swap location)?
As per the article you linked, you could run the command:
esxcli sched swap system get
On your host(s) to confirm they are not configured for swap. If they all confirm that they are not configured for swap and you have rebooted all of them, then I'd try the following:
1) Unmount the datastore via CLI
2) Check for error and check VMkernel Log
3) Post back the log anything useful!