It's certainly true that you can be vegan in a way which is very harmful to the planet- highly processed fake meats, legumes shipped across the world (causing sea pollution), air-freighted fruit, palm oil etc etc.
You need to try to think about the impact of your diet in the round, not just about whether you are eating animal products - so as locally produced as possible, without excessive energy and water requirements, non-polluting etc etc. For me, this means that I do eat local, non-commercially produced eggs , as that's a less harmful way to meet my protein requirements than shipping chickpeas half way round the world. So I'm not vegan. I have a very green cousin who eats roadkill and skip-dives for meat that has been chucked out by supermarkets, and she's not vegan either 
None of this means that veganism is in itself bad for the planet- it's how you do it that matters.