Yes, you (or, actually, the BBC) are reading it wrong.
He assumed that every single home mortgage backed by the government would default, and that every single home would have a value of zero dollars.
He assumed that every single bank in the country fails, and turn out to have a value of exactly zero dollars.
He assumed that Treasury bills would turn out to be worthless.
He assumed that the US would be on the hook for recovery programs that were proposed but never actually went into effect.
He assumed that existing programs would use every single dollar available to them under law (amounts which are, as his own tables show, many times larger than what they've actually used), and lose every single one of them.
He, in short, assumed a lot of crazy things, including that some that are cosmically unlikely, and some that are just plain impossible.
This has approximately zero chance of happening. The report, which is available at http://www.sigtarp.gov/987egapograbme123654/J09-3-SIGRTC.pdf, actually estimates US-backed assets at risk to be less than $3 trillion.