I just checked this now with 0.2.5, and there doesn’t seem to be any difference in the result from selecting content (ctrl+shift+a to select all blocks) whether or not document mode is toggled. I didn’t try the further steps, but get the drift.
I think this thread may interest you, goes in a similar direction of being able to author, edit and export ‘vanilla’ markdown: https://discuss.logseq.com/t/allow-non-outline-freeform-text/