SMP-FT is not supported as of VxRail. Yes, you can add additional nic for dell server model and create new VMkernel port for the cluster. But Internal doc says it is not supported even if you have VxRail 4.5. I don't know specific reason for this  and i'm wondering as much as you are. But this is the fact we are facing.
Is vSphere Fault Tolerance 6.x supported on VxRail 4.0 DELL Hardware with additional 10GbE NICs ? Can I create the new VMKernel adapter for vSphere FT  and setup vSphere FT using VxRail?  (1). Technical FAQ says vSphere FT is not supported in VxRail 4.0. Q: Is vSphere Fault Tolerance supported on VxRail? A: No, the vSphere Fault Tolerance feature has not been qualified for use on VxRail  (2). VxRail Network Guide says we create new vDS and new VMK adapters. [URL]  The supported operations include:   (3). And I saw past posts. Support for Fault Tolerance on any VxRail model? Re: Ask the Expert: Introducing the New VCE VxRail™ Appliance
Is vSphere Fault Tolerance 6.x supported on VxRail 4.0 DELL Hardware with 
additional 10GbE NICs?