"It's likely", when referring to something that's either true or false is a very weak statement.
Zuckerberg either knows he signed it, Knows he didn't sign it, or cannot remember if he did.
If he cannot remember, that's OK, he can say, "I have no recollection but I likely didn't sign it".
If he signed it but now he wants to steam roll with lawyers then he can use "likely" and still be legally safe.
The truth is out there but we will "likely" never know it
