The picocalc is a single cell device - or more accurately, it’s a 1s device. If you put two cells in, they’re connected in parallel and are electrically identical to a single cell with twice the capacity. The AXP2101 is being used appropriately.
That actually brings up another issue with using 18650’s here - if you put two cells in, they must be charged to the same level (within 0.1v!). Otherwise the higher voltage cell will pump current into the lower voltage cell uncontrollably. This is bad. The simplest way to deal with that is just to fully charge each cell independently with the same charger before installing them. Once they’re installed and at the same level, they’ll remain matched.