I fully understood his point and at the same time disagree with it - "open source" should not be considered a generic term, and the reasons are purely practical

I fully understood his point and at the same time disagree with it - "open source" should not be considered a generic term, and the reasons are purely practical. OSI has taken this term two decades ago and coined its widely known meaning, where "open source" means "compliant with the OSI open source definition", just as GNU did with "free" or "libre" software. The terms "open source" and "free software" have become ubiquitous in the programming world and widespread even in popular culture and redefining them brings completely no benefit - not even cultural ones, since the terms "open" or "free" or "libre" software do not have background associated with racism or sexism, unlike some of the recent nomenclature debates. And this "one group with one opinion" that you are speaking of has been one of the most powerful software groups in the world with a lot of influence on how the programming world is shaped and what software you, your family, your employers, your friends and your enemies use in your daily life, so your argument about it being "one group with one opinion" holds as much sense to me as Microsoft being "just one operating system vendor with one opinion" in a discussion about operating systems.