We know nothing. Other than that there are 1,000,001 leaked UDIDs. Everything else is just speculation and needs to be regarded as such until such time as proven otherwise.
Anonymous is not a group that can have a reputation. At best, it's a subculture. Anyone can call themselves anonymous. Whoever didn't lie in all the previous incidents may not be behind this one - this could be a totally different person or group of persons with totally different ideals.
But to assume everyone is lying until proved correct will not work. Reading news would be impossible, as would most other actives involving other people. The only working method then is to assume people are telling the truth until indication (not proof) is presented of the opposite. Blind trust would be only if we disregarded indication of falsehood when said indications exist.
A file filled with identifiers is not extraordinary evidence to link this to the claim of being a FBI laptop. It is only what it is (a file of identifiers) and says nothing about where it came from.