The first half is correct, the second isn’t 
The Ubuntu Core images are typically installed in factory processes where writing the image raw to the internal disk is a common task, the Core images are pre-partitioned and run an install mode on first boot that partitions the remaining free space of the disk…
… and since it does that, touching the existing partitions and resizing them to something not being in sync with the info inside the gadget snap (where the actual partitioning schema lives) would make the install step fail