I don’t know because I haven’t tested it.
But I’m sure you are right. The mirrors can’t come, so you’d just have duplicates.
I never consider importing the whole database. With markdown, I work with nested vaults and I’d only ever want to import part of the database. And what you get is just one file, so you have a limited number of levels to play with and again best just to import part of it.
If you wanted many small files you’d have an extra step to split it at the headings.
I simply use it as a converter. Export from Workflowy as OPML. Import the OPML file into Typora and save as a markdown file in an Obsidian vault. And reverse the process to go the other way. You could use Pandoc direct, or any other converter, but I’m most comfortable using Typora.