I have no idea what the truth is. My only point is that we (as the general public) should never take the word of spy agencies for anything, especially not about what their perceived enemies are doing.
Perhaps what they said happens to be absolutely true. Perhaps they have withdrawn their diplomats for fear of other attacks (e.g. poisoning, as Canadian authorities have apparently claimed). Perhaps they have withdrawn their diplomats in order to create some impression that they are in danger. Perhaps they have withdrawn their diplomats because they intend to do something that they think could put them in danger.
There are myriad possibilities, but until we get a chance to see classified reports or we get some independent reports about what is going on, we shouldn't speculate based on nothing more than the claims of spy agencies.
Edit to add: if the story would have been "Cuban intelligence agency reports how US diplomats faked symptoms to claim they were being attacked", would you believe this claim? Even as a Cuban? I for one would not - and just as I wouldn't believe this claim, I will also not believe the opposite claim from US intelligence.