It's a calculated trade-off. We assume that NetApp does the right thing and crunches the disks once they have done a failure analysis back in their labs. ---
If you assume that, and you really care, I would not assume any vendor of drives or storage material does this.
I have NO idea what Netapp does, but I would not ASSUME that they physically destroy disks with your data on them.
Right now, I assume youre thinking about upping your service contract to be able to keep your own dead disks so that you can destoy them yourself and remove the assumption of the mitigation of risk.
But only you can know for sure.