Taking a look at the website itself, I'm not really filled with professional interest. Her credentials are minimal at best, and the explanations on the site about this particular stretch are all very specific to what you may or may not be doing. Things like keeping your spine straight and your pelvis level are still possible with the forward lunge- and, they may also be advisable.
However, my main argument would be this: I'm not seeing any indications that this "replacement stretch" is going to bring us closer to something like the front splits, nor am I seeing any indication that it would be an appropriate stretch for the hip flexors. It's very poorly explained in these ways, and as the post moves into talking about this "replacement stretch," it talks more about strength training, rather than flexibility.
The hips, and the rest of the body, are complicated things. What can be good for some people are not good for others. I have heard lots of people say that you shouldn't roll your head in a complete circle, I've heard that running will cause my knees to feel even worse (since running, I have been out of chronic pain for a total of four months), I have heard that squats that bring your hips lower than your knees are doing absolutely nothing, or may even be damaging.
Few, and probably none of these things are true. But, we will always get conjecture and speculation, "advice" and recommendations as to what we're supposed to be doing or not doing with our bodies.
However, I would be more inclined to trust results and science, than all of that. Thousands and thousands of ballet dancers, as well as other dancers (and other people besides), around the world, use the forward lunge stretch, and I have heard very little in the way of adverse effects, unless the person does it incorrectly, too hard, or has a particular issue with their hip.
So, that's where my trust goes.