Can an Apple device be erased if it is locked with Activation Lock (Find My iPhone)?

Yes, an Apple device can be erased even though it is locked with an activation lock.

Find My iPhone includes a feature called “Activation Lock” to prevent someone else from using the iPhone, iPad, or iPod touch if it is lost or stolen. The activation lock will be automatically activated when "Find my iPhone" is enabled for a device with iOS 7 or higher.

If "Find my iPhone" is turned on for the device, it can be erased, but it can not be activated after erasing while "Find my iPhone" is enabled in iCloud. The device polls the previous owner's Apple ID and password during the setup and activation process. Further information can be found here: https://support.apple.com/en-us/108794