When you chunk it you have problems about what happens if that process is interrupted. So it's not trivial (though solvable) but it's the kind of atomics you want the new engine to do.
I think the person you're replying to is saying that the document should be split across keys inside the implementation, i.e. split across the fdb keyspace, not split by the user at the application level. Which is the approach you mostly always have to use for 'large' values; FoundationDB has size limitations on the k/v pairs it can accept and splitting documents and writing those chunks in small transactional batches is the recommended workaround (along with some other 'switch over' transactional write which makes the complete document visible all at once.)