@Dell_user_999 I think you are required to have a Thunderbolt 3 Controller. Note the following statement in the PDF, "While all systems with Thunderbolt 3 Controllers have a USB Type-C port, having a USB Type-C port does not ensure that you have a Thunderbolt 3 Controller ". Also, note the statement below the one I quoted that reference a Dell knowledge-base article stating that a Thunderbolt 3 Controller is required to use the Dell Thunderbolt Dock. 

Thank you @Dell_user_999 for your input - that's quite a possibility.

After further searching the XPS 8910 Guide and Specifications, the following is the excerpt regarding port (5):

Port (USB 3.1 Type-C port)
Connect peripherals such as external storage devices, printers, and external displays. Provides data transfer speeds up to 10 Gbps.

I am confused... it actually mentions that this port (5) can also be used as an 'External Display' among other devices, even though, seemingly it is not Thunderbolt 3 compatible. I also wonder why all (5) USB-A and audio ports are functioning perfectly.

Is there a way to make that USB-C Thunderbolt 3 compatible?

WD15 is not a thunderbolt dock.  It uses USB type C for Data Link and it should be compatible with your XPS 8910.  All ports including video ports should work, but only one 4K at 30Hz..

If any port is not working properly, check with seller for an exchange.  Search on the zone, these are around 20 usd with 180w power adapter.

The WD15 may not be a Thunderbolt dock but the PDF states that it requires a PC with a Thunderbolt 3 Controller. I am pretty sure the XPS 8910 does not have a Thunderbolt 3 Controller and I don't know why the PDF says the WD15 is compatible with the Special Edition of the XPS 8910.