It's not about copyright or the OSI's lack thereof. It's about abusing language to bring along a mindshare that isn't rightfully yours. The OSI definition of open source is the broadly accepted definition of open source. Using it to describe something which isn't is lying, in the same way that serving someone bacon and eggs when you put "cake" on your menu is lying. Neither the terms "bacon and eggs" nor "cake" has to be copyrighted for this to be true.

Not that I'm acusing the author of DevHub of lying, I'm just clarifying the problem.