There are two (more than two, but let's stick with two) basic views of how we define concepts. One is that we define concepts by definition. A cofounder is someone who helps to found a company, therefore (by logical definition) an already-founded company cannot acquire more cofounders. The other is that we define concepts by comparison to bundles of referents. For instance, we know that Steve Wozniak, Paul Allen, Paul Graham, and Gordon Moore are all called "cofounders", and that they worked their asses off for equity. If your startup is looking to hire someone who works and gets paid just like a cofounder except without actually cofounding the company, it's looking to hire someone who resembles our "cofounder" referents even if it doesn't fit the technical definition.
Hence, this controversy just comes down to different ways in which people define and use concepts. Personally, I find it useful to be able to think in both modes.