The Write cache configuration available with the "isdct set" command is for ATA devices only. It does not apply to the 750, or other PCIe* NVMe* SSDs.
The Intel® Solid-State Drive 750 Series are PCIe NVMe* drives, so they are different in many ways from previous generation SSD's. Write cache on this type of drive does not work the same way as previous SSD's and it can't be configured by the user.
You can find more information about this command in the Intel® Solid-State Drive Data Center Tool User Guide, page 20.