I'm a bit confused by this story. He says he found it 'abandoned' in the backlot of some AZ aircraft graveyard in 2009, but it was actually loaned to the Pima Air Museum in ... 1993.
It was on their lot, it just wasn't on display at that time. Probably because there were already eight others ( plus some wrecakge ) on display around the USA... Despite its secrecy it's not a rarity.
Also the serial he states is incorrect, 90-0533 is a USAF C-17A transport; the 90 indicates the Fiscal Year in which the procurement was authorised. That D-21's serial was just a non-standard 533. Like a lot of the A-12 / SR project things weren't always booked and accounted like they should have been.
There's also a D-21 out for all to see at Blackbird Air Park in Palmdale (which has plenty of other awesome stuff as well, such as an A-12/SR-71 - don't remember which airframe - wind tunnel model).
Is this a similar concept as the cancelled Soviet Lavochkin La-350 ramjet cruise missile design of the 50's, which was based on ideas of a German Dr. Wolf Trommsdorf (who was working for the Russians after the WW2)?