I think you will be surprised at how quickly that routine becomes easy.
The flaw is that it is the same number of reps, with the same weight (your bodyweight), repeated over and over… Therefore your muscles will adapt and increase in strength/size to compensate to that stimulus, but then stop.
So go ahead, it won’t hurt you, you won’t develop a big gut. The trick is, after it gets easy and you have adapted to it, get a more difficult routine. When the routine is really, really difficult, your body may require additional days rest.
However, it’s general consensus (mostly), that you can train abs A LOT.
CT, (whose routines you might want to try after you get through with being a beginner) advocates a much more intense routine for abs in a lot of his workouts, and abs are worked 7 days a week.
Personally, I’ve been gradually increasing my volume and intesity of core training over the last year and am now doing 4 days a week very intense, plus 2 lighter days.
perhaps i should note that the 15 years of training previously, I have never improved my abs as well as in the last year: and the reason is that I am increasing output with every workout.