So it’s made of shit, right? And shit is an animal product. But barring a night of drinking or a particularly aggressive burrito, shitting does not harm the shitter; it’s beneficial and required. Also the animals in question can and do consent, does that make it vegan?
It depends on why someone is a dietarily vegan. If someone is such because they need to avoid something in animal products, like cholesterol, or an allergen. Impossible meat would be fine. If they are “whole foods, plant based” then they probably aren’t going to eat an impossible burger unless they don’t have better options. There’s a whole spectrum of reasons people go vegan, and most of the ones I know have a combination of moral, health, and environmental reasons.