The gist I got from reading this was the Mac Mini was serving files to employees at the ad agency, yeah? It doesn't seem that unreasonable to me either that this would be sufficient for the task (depending on the size of the agency).
What's missing from this article is what the service provider wanted to charge the ad agency for a new server. An M2 Mac Mini is like $600 USD. I just Google>Shopping searched "2u file server" and the prices for new equipment seemed to start at around $4,000. Is that close to what an IT company like the one from the article would charge or would there be some significant markup?