The documentation of the input 'selectable' is the wrong way round with MatChip and MatChipList:
MatChipList
Whether or not this chip is selectable. When a chip is not selectable, its selected state is always ignored.
MatChip
Whether or not the chips are selectable. When a chip is not selectable, changes to it's selected state are always ignored.
https://material.angular.io/components/chips/api