For special requirements (allergies, diets) reengineering the patty would be way easier than reengineering a cow, i would expect. This is their first product.
Allergies, yes. Zero carbs, probably impossible. That's okay though. No product can satisfy everyone!
With ~5g of carbs per 200 calories, you would have to eat these things pretty much exclusively to break keto, the only diet I'm aware of with an actual need for virtually no carbs.
I'm guessing someone on keto would need at least 150g for satiation, if not double that. For reference, a quarter-pound is about 120g. That's about 10.5g of carbs. It doesn't break the bank by itself, but it does mean there's a limit to the amount of extras on the burger.