It makes sense, though. Hashirama was and will pretty much always be known as the ultimate shinobi. To call him the God of Shinobi is simply a hyperbolic re-phrasing of his actual status. Likewise, Hiruzen was most likely the definition of a model shinobi to those of his era/generation when he was Hokage, therefore earning the nickname God of Shinobi as well. Both can share the title, nothing against that.
Although I can see where you're coming from, now that you'll have to deal with Hashirama fans attempting to discredit/counter any form of a "God of Shinobi"-type argument.