The Valovi Chair chair calls for 3/4" or 18mm B/BB grade Baltic Birch. All Opendesk products call for that material or what would be called 1/2" or 12mm. You can see in the images on their site, every product uses one of those materials, except in the instance where a laminate is on top.
When you get this material back in your shop, it is up to you to measure the material thickness and determine how to alter the DXF based upon it's thickness. You'll notice on the Valovi Chair, it has slots for the support pieces to fit into. You need to measure that specific gap in the drawing and resize the entire drawing based upon your wood thickness. For me and my CNC machine, I typically resize the drawing to be the same size as my wood thickness. This gives me the proper friction fit. This is something you will need to test for yourself based upon you CNC machine and machining method.
If resizing the the entire drawing makes the product too big or too small, you potentially need to find an alternative source for your material.