I received an official statement from someone at Anker.
Despite using a USB-C port for input, the powercore+ 20100 is NOT power delivery compatible. Anker strongly advised not to use it with either of the MacBook chargers!
Kindly be advised that this Anker PowerCore+ 20100 USB-C doesn't support Power Delivery technology via its USB C port. It's USB C port is more like a physical port to connect Type-C devices. To recharge it, we always advise customers to use a 5V/2A wall charger (such as the one that ships with the iPad or Galaxy Note).
Our product manager did tests using an Apple 87W/61W wall charger with this Anker Powercore and found the battery is not charging properly at all, judging from the current & voltage his device recorded. So we strongly recommend you to use a 5V/2A wall charger only to charge this battery. It is not safe to use those powerful chargers on it.
@ShaColby_Jackson, you might like to confirm this advice with them, given you do charge using the larger MacBook wall charger).
I'm personally very surprised that a device which uses USB-C as an input isn't Power Delivery compatible. Using a connector that easily allows it to be attached to a charger in a way that is unsafe seems incredibly risky.