Comparing it to this photo of a 280 mm from Wikimedia commons, the shells behind the howitzer look to be about the same size relative to the people. They certainly don't look noticeably bigger to indicate a 350 mm diameter.
"Sometimes things that are not true are included in Wikipedia. While at first glance that may appear like a very great problem for Wikipedia, in reality is it not. In fact, it's a good thing." - Wikipedia.
If you look at the left side of the image there is a church spire on the horizon. This looks rather like the Eglise Saint-Martin d'Harfleur which is still
there. 3D in Google maps shows off the spire rather well.