WinUSB is a Microsoft created/supplied driver.  All Microsoft-supplied drivers contain an embedded signature from Microsoft. 

In operating systems prior to Windows 8, WinUSB-based devices require the user to install a driver package for the hardware. However, starting with Windows 8, it is possible to make WinUSB-based devices that are fully "plug and play", and do not require any user-supplied driver package. Windows 8 allows for automatic installation of the WinUSB driver, when the device firmware implements the correct Microsoft specific "OS" and related USB descriptors. These special descriptors are optional, but when implemented, allow for automatic driver installation using the WinUSB driver that is distributed with the operating system installation.

When used with Windows, the CDC projects in MPLAB Harmony use the Microsoft created/supplied usbser.sys driver. This driver contains an embedded signature from Microsoft.

These USB device classes/projects rely on Microsoft-supplied drivers that are distributed with the operating system, and do not require any user-supplied driver packages or .inf files. Therefore, driver  package signing is usually not relevant for these types of applications, as the drivers are normally installed automatically when the hardware is attached to the PC.