Does adding salt when soaking dry beans toughen or soften the skin of the bean?

I have heard opinions on both sides of this. The the latest answer on this site, says salt softens bean skins quoted Cooks Illustrated (I can't see the article, but I trust CI), while another answer says salt hardens the beans' skins.

In case is varies by bean, I'm interested in:

• Kidney Beans
• Black Beans
• Small Red Beans

So which is it?Inquiring Minds want to know.