I'm a vegetarian who's cat has eaten meat all her life - I've also had health problems that have forced me to learn alot about suppliments, nutrition etc.
I've only had a glance at that website, and I've never done much research into vegan or veggie cat food. The fact is that you CAN get protein from many things besides meat, but sadly beans, pulses, soya, nuts, cheese etc etc all have DIFFERENT types of protein - both to eachother, and to meat proteins. Human veggies and vegans are advised to eat as broad a spectrum of proteins as possible, just to make up for the lack of meat proteins in our diets.
And we evolved as omnivores!
I would be VERY careful of giving any naturally carnivarous animal like a cat, any food but... well, meat, really. many people feed their cats and dogs pure raw meat diets and say its the heathiest thing going.
Still, if you were hell bent on forcing your cat or dog or whatever to eat veggie or vegan food, I would say that to do so responsibly you MUST research it as much as possible to be confident - not just whats in the food and what the company tells you, but the biology and diet of the animal down to the base proteins, enzymes, vits&mins and everything else that it requires to function healthily.