Update from my end:
AGR Insider has written here and in PM to me that this is real. Select Plus members can access United Club, both old RCCs and Presidents Club.
United called me after I wrote in and told me the email by Amtrak is a mistake. However they are starting to honor the mistake. I told her what AGR Insider told me that this is not a mistake but didn't push too hard given they said they are honoring it. I also told her I know people who have been refused entry more recently.
I will test this again in Dec. I don't hold breath it will work. I did get compensated by United but it's all annoying.