It's complicated, but here's the essence of Binker's exposé: Thanks to the financial crisis of 2008, investors in the two mortgage giants were mostly wiped out. Fannie and Freddie were then taken over by the federal government, and big hedge funds swept in to scoop up the companies' depressed stock, "essentially placing a bet that the companies would rebound under federal protection," as Binker puts it.
The proposed reform legislation (which his bipartisan support and the backing of the White House) would let the courts decide what should happen to those current investors, and their bet may not pay off. The Koch ad makes it seem like ordinary folks with their modest pension funds would be at risk, but as Binker explains, those sorts of people no longer have any exposure. They took a serious hit in 2007 and 2008, when Fannie and Freddie collapsed, but their investment funds have long since sold off that stock. The current owners are now those large institutional investors who, again, made a speculative play when the two companies hit rock bottom.
And yes, those guys could get screwed (oh, the humanity!), but teachers and firefighters are not among their numbers, despite what the Koch ad claims. When the spot first came out, we didn't have the benefit of Binker's detailed analysis, but we did say "it's a safe bet" that the Koch brothers' "personal financial interests are at stake." Now we know that that's almost certainly the case. When it comes to the Kochs, it's almost always a sure thing that they're peddling b.s. in furtherance of their own private agenda.