I think the response would have been different if the design firm had openly approached the Wikipedia community at the beginning of their project to offer their pro bono services. Instead it seems like the designers did the work in isolation for 2 months and then sprung it on the world--not a very collaborative working style.
I believe most design firms offer some kind of prototype as a first step, which is what they did. They weren't commissioned to do anything, and there is no go-to person at Wikipedia for them to work with; an iterative process would have required working with a democratic committee of volunteers who individually don't have the authority to make any kind of decision.
I've hired a lot of web design firms over the past decade and I've never seen or requested a 2-month-long prototype phase as the first step.
The first step in a web design project is always discovery, so that the design firm does not waste their time (and my employer's money) on work that does not solve the right problems, or cannot be implemented.
Ad agencies, in contrast, will typically come to a pitch meeting with some concepts or prototypes. But that is ok because ads are self-contained products that are largely free from legacy systems.
Wikipedia is fairly democratic, they're open to redesign decisions if they work for the site and are free. See this response to the article: https://en.wikipedia.org/wiki/Wikipedia:2012_main_page_redes...
Of all brands the designers could have chosen, Wikipedia may be one of the most appropriate.