They designed the puzzle using a Java applet at Puzzle Laboratory, which lets you specify the precise set of clue squares and will then attempt to invent a soluble puzzle starting from that.

I haven't yet worked out how it's done, though. It certainly isn't as simple as inventing a random filled-grid solution, blanking out all but the specified clue squares and seeing if the result is soluble, because I've tried that and it never seems to get anywhere.