I don't think you should use that article as something to say being vegan is bad for health. First of all, most of the evidence to say it is bad is “I just didn’t feel good or healthy”
The strongest argument I could find was this which is given in the context of weight loss,
“It’s hard being a vegan to eat enough good, quality protein and not have too much starch. I know a lot of fat vegans,” said Dr. Hyman
However that statement is only true if you know nothing about nutrition. It's very easy to get enough protein and still come way below your daily calorie requirements. But you don't need to believe me, you can look at the evidence yourself.
Go to chronometer
Add lentils, chickpeas, pasta, nuts, beans, and even things like oats and you will see the protein % intake overtake the calorie intake every time. Those are the calorie dense foods too.
Try adding in brocoli, tomatoes, etc and you will find even then that the % protein overtakes the calories you consume. Not that I personally want to eat 1700g of broccoli to get 48g of protein but I mean you could. And it would only be 578 calories and you would be stuffed, assuming you could even manage to eat that much. Not only that but you would get all of the vitamins and such in the broccoli which is far far more than 185g of a t-bone steak(almost the same protein amount).
Anyways the best argument is simply knowing what you need, what you don't want, and knowing what foods have what. It's not even an argument, it's simply getting what you need to be healthy. Chronometer makes it pretty easy and while it's not 100% complete it will offer you far more info than 99% of people know and you are certainly better off with it than without.