This module works well once its input requirements are understood.
It wants two curves that are not connected, so in my case, I divided one of the lines into >100 points and just deleted the point closest to the second line and then rebuilt the curve with INTERPOLATE CURVE.
The advantage of this module is that the fillet curves are available as a separate output. The only way to get these from FILLET is to CCX with the input curves. Unfortunately you run into the limits of tolerance of the CCX module operation with fading fillets rendering the FILLET module useless if you want anything other than joined filleted curves as output.
The secondary benefit to using this module is that the curves do not need to be JOINed - an operation that inexplicably shuffles all the curves in a seemingly random manner requiring a sort operation.