He signed a piece of paper to affirm the opposite.
People don't put their signature on written things they believe to be bull$h!t.
If you read the letter, it never actually said the laptop was Russian disinformation. They just said it had "all the hallmarks" of Russia disinformation. And they added a disclaimer at the end of the letter saying that they weren't saying definitively that it
was Russian disinformation.
Of course, being intelligence types, they buried that little disclaimer deep in the letter after they knew anyone reading their letter would already have concluded that the 50 were saying it
was actual disinformation. Which is exactly what they wanted them to conclude.
They misled people deliberately so as to give themselves an out later. Transparent as hell.