I don't think anyone who really wanted to get into the US would have a problem handing over A social media password. Whether that's a password for an account specifically created to be reviewed by immigration authorities is another matter entirely.

What makes DHS think they would be reviewing a real social media account?

I think this is not about reviewing social media accounts. This is about giving DHS and CBP a pretense to deny entry to anyone they feel like, based on gut feelings, prejudices, whatever.