Your question really doesn't make any sense.
Say there's a box and someone is claiming there's a ton of gold in it. And say my position is that there is no ton of gold in the box.
So I try to pick up the box and I succeed. I pick the box up. This proves there isn't a ton of gold in the box because I can't pick up a box that weighs a ton and a ton of gold weighs at least a ton.
My argument would be, "Why can I pick up the box?"
Now, you're coming along and saying, in effect, "But if I could pick up the box, that wouldn't prove that there is a ton of gold in it. So why am I making such a big deal about being able to pick the box up?"
Do you see why that makes no sense? I'm not the one trying to prove there is a ton of gold in the box, I'm the one trying to prove that there isn't. And once I've found such a proof, why the heck would I, or anyone else, care what might or might not prove there is a ton of gold in the box? We already know there isn't because we can pick the box up.
If it doesn't, then why do websites such as why won't God Heal amputees? exist?
Because those who claim there is a god have to explain why god has never healed an amputee. If they can't do that, it's an argument against their claim.
What would or wouldn't be the consequences of your counterfactual world were amputees regrow limbs has no relevance because we don't live in that world. But yes, you're right, in that world, that amputees regrow limbs would not be evidence of god or the supernatural.
This is because terms like "god" and "supernatural" are only used for things we know are impossible. We literally call something "god-like" when it does things we know are impossible. We call something "supernatural" when it displays abilities or properties that contradict our understanding of the laws of physics or our notion of cause and effect.
Those kinds of claims will never be provable and sane, rational people will admit that they know that they require things they know are impossible and so are themselves impossible.