2018 MacBook Pro + Dell U2718Q, single cable for display and USB data

No.

Technically speaking, you have a Thunderbolt 3 Type C port. "C" is the physical charastic of the port, not the USB specification. Being that it's Thunderbolt 3, its carrying several signals:

• PCIe
• USB 3.1
• DisplayPort
• Power

Your "end" device is a monitor that has several different inputs

• HDMI
• DispayPort (DP)
• Mini DisplayPort (mdP)
• USB

When you go from your MacBook's TB 3 port to the video in, you're not passing the USB signal; this is why you need a second cable for the USB functionality. If this was a monitor that accepted Thunderbolt as an input, you could do this with one cable.