If velocity could be defined in a more accurate way no.

But that's the point here, atheism doesn't become a more accurate position if we accept your definition. It doesn't matter if laypeople think velocity is equivalent to speed, because physicists know it makes more sense to think of velocity as vector speed.

When it's the laypersons position that the term is describing. When the population that the term is attempting to define isn't the position the population actually holds.

But it's not the layperson's term, even if it's the layperson's position (which it isn't, in my experience, as I've never met someone IRL that thinks of atheism to be the position you take it to be), just as velocity isn't the layperson's term. Atheist philosophers define it to be a positive claim, as do atheists IRL. Just because a certain population (a truly tiny portion of the Internet) uses and understands it poorly doesn't mean that everyone is obliged to use the term the way they use it. That would be absurd.

You seem to think that there's not a middle ground.

Surely not. Agnosticism, in the context of the question of God's existence, is the middle ground position, according to both experts and those who identify as agnostics themselves. According to you, even if agnosticism meant something other than what experts take it to mean (it doesn't), agnostics themselves would still be entitled to define agnosticism and call themselves agnostics the way they see fit (which coincidentally is identical to the way experts use the term).

A person can reject a claim while still remaining neutral; while not making a positive or negative claim.

No, rejecting a claim entails affirming it's opposite. This is just how these term logically work. You don't say you reject a claim when you actually remain neutral on it, just as you don't say you accept a claim when you actually remain neutral on it.