Why? It's not like they filmed the movie on the actual battlefield. Their interpretation is no more accurate than ours.
That scene was infact filmed in Belgium near passchendale
IMDb says the movie's battle scenes were filmed in Canada.
Wiki says
Passchendaele is a 2008 Canadian war film, written, co-produced, directed by, and starring Paul Gross. The film, which was shot in Calgary, Alberta, Fort Macleod, Alberta, and in
Belgium, focuses on the experiences of a Canadian soldier, Michael Dunne, at the Battle of Passchendaele, also known as the Third Battle of Ypres. The film had its premiere at the 2008 Toronto International Film Festival on September 4, 2008, when it also had the honour of opening the festival,[2] and it was released widely in Canada on October 17, 2008.