I just got a nexdock xl today. Charged the xl battery then plugged in the L5. Nothing happened at first. Then, I rotated the usb-c cable when plugging into the L5 and it docked 
After about 10 minutes of use, I notice the L5 is hot, and charging stopped. Otherwise it was charging from the xl.
The Mobile Settings / Convergence screen doesn’t show that it is docked to the xl though? That is disappointing. I will eventually reflash the L5. Some things have gotten a bit haywired on my L5 of late. Not sure why. Maybe that will help.
As I test/explore more with the xl I’ll post here.