No commercial usage permitted from what I read on that site.
There is substantial discussion about this in the comments section of the article. Museums cannot claim copyright to public domain works. Public domain means anyone can use it as they wish, commercial or otherwise, because the copyright has expired.
However, some online archives may ask you to agree to terms of service before accessing, so they could try to restrict your use of the images that way. That could hold up in court if the terms are explicit and clear, and you're actually clicking an "Agree" button or something to that effect. It is not enforceable if they're just tucked away on a "Terms of Use" page somewhere on the site.
Also I AM NOT A LAWYER, this is just something I've dealt with numerous times before.