Back in 2008 I was doing research for a story in Men’s Health when I came across a particular journal article that helped influence how many people train now their abs. The study revealed that planks activate significantly more of your rectus abdominus (the six-pack muscles) than crunches. In fact, this particular study (which was published in the Journal of Strength and Conditioning Research) found that planks cause 100 percent activation of your six-pack, whereas crunches only led to 64 percent activation.
Source: Are Planks Overrated?

What? No Variety?
From the contents of the article it almost seems as though people who are fully capable of utilizing their abdominals for a proper plank ought to continue what they are doing…
The entire premise seems hinged on the anecdotal observation that people lack core abdominal strength. What really bothers me though is that rather than address the actual problem this coach is suggesting people just do push-ups instead. Done correctly a push-up can be an excellent abdominal workout; but only if you’re able to engage your abdominals while you do them. …so essentially this person’s advice to people who lack strong abdominal strength is to do another workout that (to do properly) requires abdominal strength. All they are going to get out of the push-up are some vanity muscles.
Another aspect that irritates me is the idea of picking specific yoga postures, considering that vinyasa/sun salutations incorporates a tricep pushup. They could do with a little less cherry picking and ego for those who don’t want to be associated with yoga.