I’m not a regular DEVONthink user, so others can check this, but my understanding is copying items to DT does pass Tinderbox $Tags into the DT tags, as of Tinderbox 6.4.0 and DEVONThink v2.8.8.
My earlier point - this seems the only way MarginNote has to export such data which seems pretty inflexible.
In the (private) thread I mentioned I think the MN user is manually putting the ‘#Tags’ into the exported text files and I’m helping him remove these from $Text, clean off the MN clutter (hashes, underscores) and place it in $Tags.
Even in the Pro MN version (needed for most export types) looks pretty limited for export and is tied to a few target apps some of which require a further third party app en route, e.g. you need OmniOutliner to get to DOCX (according to the MN manual). I can’t check further as there’s no user forum. Frankly, I think it would make far more sense to get the MN dev to offer a CSV or Tab-delim output which Tinderbox and other apps can easily handle rather than push Tinderbox for more niche and breakable (each version of both apps has to be tested for unintended breakage) bespoke export/import. My 2¢, anyway.