Can't the company be compelled to provide salary data for the role? I assume there is some kind of enforcement mechanism. So then it's a matter of explaining why your lowest paid senior engineer makes 150k, the median makes 200k and you've listed a floor of 10k? The judge/jury don't have to accept a smarmy response about how the hypothetical candidate might conceivably make 10k.
And on the subject of the window, most places have much narrower bands than that for a role. It might be that wide for all engineers, but not for senior engineers for example.
An 60k is easily an entry level salary. And 300k is easily a principal engineer's salary. Put the two under the same job title and then you can say to the jury, "it's not just credible that this salary range could exist, this range actually already exist right now."
They do at my company. Everyone is just "Software Developer". I don't see anything wrong with it, I find that titles are largely meaningless anyways. At some companies "principal engineer" is handed out after 3-4 years of experience, and at others principal engineer is the highest title with only 1-2% of devs holding it.
And on the subject of the window, most places have much narrower bands than that for a role. It might be that wide for all engineers, but not for senior engineers for example.