Snapstinget, the article says 'cross training, such as cycling, strength training, rowing or elliptical training'. I'll be aiming for cycling and rowing, maybe with a bit of weight training.
I guess that high-impact activities with rapid changes of direction, such as squash, might not be a great idea as the emphasis is on reducing injury risk; and minimal-activity classes like yoga might not have much effect on your marathon time (except by reducing injury, perhaps) - but I don't get the impression that there are hard and fast rules beyond common sense like this.