Behaviour isn’t sufficient to be vegan. A leaf-cutter ant is not vegan, they just coincidentally act in way a vegan might. Likewise, a human stranded on an island with no other animals that they might prey on is not necessarily vegan. However, being in a situation where your life depends on animal consumption (vaccines come to mind as an example we all have to wrestle with) doesn’t automatically make you not-vegan. It’s an ethical philosophy. If it’s not being practiced mindfully, it’s just a coincidence.