From the thread, the guy explains he studied every page of the manga where the workshop is and worked from there with other sources of information to have the correct tools for the job/era.
Regarding the galerie, he tells us the mangaka created it from multiple ones, the guy doing the lineart for the anime recreated it from that source and completed it with books about galeries and again tried his best to exclude anything anachronistic.

For outdoors, he worked from postcards/pictures around the time the anime takes place. They had to remove some monuments/buildings at times... Well they tried their best.
I think it's great a japanese team decided to have a french team work on such important parts. Even if it can not be perfect in the end, at least the quality level clearly improved.
With that thread, I learnt those guys work for other japanese projects.
Thing is, France is the second Japanese Manga market after Japan... it's not unnatural to see joint project I guess. It's just that with that anime, it becomes a lot more concrete to me.