How to beat Laura Poitras publishing a public key all over the place?
Without forward secrecy, getting Laura Poitras' key will enable the NSA to read all past communications. They only have to seize her computer when it's still on, and the key is still in memory somewhere, or compel the poor journalist to give up here keys (possibly using that "non invasive" waterboarding torture, and justifying it with suspicion of helping terrorists).
Now if Laura kept the decrypted messages in her laptop, forward secrecy wouldn't do anything, but if she properly deleted them, it would be a shame if the messages were nevertheless at the mercy of the attacker.
As for key finding, well… the simple solutions do work pretty well. Snowden for instance didn't find Poitras' keys lying around on the internet. He asked someone he trusted would give him the right key.
It isn't real ambiguous.
For instance, Snowden had someone tweet a key fingerprint: https://theintercept.com/2014/10/28/smuggling-snowden-secret...