This question is relatively old as the Slate AX has been out. I just purchased mine. My first question was why the “big” power brick. Reading the comments here I am a little surprised by some. Such as PD is for batteries. Which is outright wrong. Both my Dell laptop and MacBook Pro are powered by PD over USB-C ports. Note both are also Thunderbolt ports. I use a Dell dock for the Dell laptop and a new GaN 200W charger for the MacBook Pro. The latter charger has a display showing how much voltage and wattage is being provided on each port. I plugged the Slate AX into the 2nd USB port and you can see the results in the picture below.
I also decided to try my 240W battery with 4 USB-C ports to power my new Slate AX. The battery also has a display but it only shows wattage being delivered on the port, . It shows 3-4W being delivered just as the wall charger in the picture. I don’t know how that works out into voltage and amperage but I assume it is the same as the wall charger.
I don’t see any reason why a USB-C PD charger won’t properly power the Slate AX. It would be nice to know officially if the Slate AX works with a PD charger. As in does the USB-C port actually support PD.