I took a few photos.
This is the frame hinge from the inside:

Dahon Mariner D8 2024 frame hinge

An odd thing happened: while taking the photo, I randomly turned the screw, and the hinge is now seemingly.. Fine? I don't get it. Twice I turned the screw all the way one way, then tightened it in small increments, and at every point, the hinge had a gap when latch was clamped.
Now all of a sudden it works..

But also, just for future reference, posting images on how Deltec on this model is attached. I found these attachments odd from the very beginning, as the attachments points themselves are obviously weakest points.
So this is Dahon Mariner D8, but I have Shimano gears and derailleur, and my seat is sports seat and not the one usually depicted, and the tyres are not what is usually listed, and there is Deltec on this one, and the hinge is as you say newer. I have not been able to find this exact configurations anywhere. And I'm sure bikeshop swapped some parts. It might have been a returned bike or something, but they did not disclose this.