Remember the Britam hack? Private security company Britam got hacked, tons of internal documents released. Among them one "smoking gun": that the company had allegedly been offered a contract to stage a fake chemical attack in Syria.

It was just that this mail had fake headers, transparently copied from another mail in the same archive.

There was also another attempt to plant a false flag narrative in another hack, of the private mail of a US officer in Egypt.

This Guccifer has alreadly bumbled once by (apparently) not being aware of all the metadata that leaks into MS documents. We know they use Russian keyboards... So, you're right to not trust them by default. But also, it's too easy to dismiss anything they leak - faking data is hard.