I don't think the intention was in any way to deceive the audience. I personally see it as part of their post-modern filmmaking[0] style.
What personally makes this style interesting to me is that there are usually several layers of meaning and intention. And usually references to other films or common perception or convention.
Even if the intention is to tell the truth or a true story, there's always the bias of the storyteller, the selection of what to tell, and what not to mention etc. So in this case, the Coen brothers re-used the "based on a true story" concept, and even exaggerated it. They re-use the format, but mock it at the same time.
It doesn't stop being an attempt to deceive just because it's self-aware, ironic, or motivated for other reasons. Likewise, murder during a robbery is still murder even if the primary is theft.
