No, if you remove all imported content and then re-import, it will bring over everything that's currently in your LJ account. That sentence in the original post means that if you had (for instance) an entry that you'd posted on LJ, imported into DW, and then deleted on LJ, and you run the tool to delete imported content, the entry will be deleted on DW and then that entry can't be reimported to DW, since it's been deleted on LJ and we use the current contents of your LJ to import.

If your comments won't import, check your inbox for errors, and if there are no errors, open a support request.