The problem with this is that it's mainly a disadvantage to disclose your current salary if you're earning below market rates.
If you're earning above market rates you want to disclose your salary both to show the company how much your previous employer values you and also to get you a higher salary at the new company.
Hence the people who aren't disclosing their salary are implicitly informing the company that they're currently being paid below market rate. While it might not be true for everyone who refuses to disclose salary, it's true for a large enough percentage of applicants that it's safe to assume.
If you're earning above market rates you want to disclose your salary both to show the company how much your previous employer values you and also to get you a higher salary at the new company.
Hence the people who aren't disclosing their salary are implicitly informing the company that they're currently being paid below market rate. While it might not be true for everyone who refuses to disclose salary, it's true for a large enough percentage of applicants that it's safe to assume.