Hello.
Current signature of importCells is:
importCells: (cells: Cell[], dx: number, dy: number, target?: Cell | null, evt?: MouseEvent | null, mapping?: any) => Cell[];
In the "Drag Source" example importCells being called inside DropHandler for gestureUtils.makeDraggable. It feels like its common case and current implementation makes you write something like:

const xNum: number = x || 0;
const yNum: number = y || 0;

in every TypeScript DropHandler implementation.
Original mxGraph docs says that default of x/y args in importCells is 0, so my suggestion that x and y should be optional in it's signature.