This only matters if your employer cares enough to choose to create negative consequences for you. I guarantee almost no employer cares about your vim configuration technically being their IP. Even the lawyer work to figure out if it's their IP or not costs more than any possible gain they could have from claiming it.

If your employer is extremely spiteful, they might burn a pile of cash to hurt you. But that's not normal.

Or in the situation described

>it got checked into the repository and now the script they used at their last job too has their current job's copyright notice and license on it, and they're perceived as being petty for trying to claw back their own intellectual property

Where someone is causing a fuss trying to claim ownership of something they never actually owned and thinking the other people are the ones being unreasonable.