The criterion is the development process, not the complexity. Linux is complex, but not a Cathedral.

I don't want to split hair with your words more. For context, FSF hard liners since the dawn of the OSI were distorting the meaning of CatB to deflect criticism from themselves. FSF supporters also very successfully promoted "FLOSS" instead of bare _OSS, giving lots of later-comers the illusion that "free/libre" was an expansion pack for OSS when OSS came later, a very intentional evolution of the dogmatic "free" software movement.

The choice of "Cathedral" is an extremely obvious symbol when you consider the Protestant reformation as a defiance of Vatican, an overly central system where decisions can only flow from the top. There are a lot of metaphors ESR could have chosen from, but the "cathedral" rhymed with the undertones of the real tension between the many OSS practitioners who have divers motivations and the FSF's plan to slap GNU stickers on every piece of software on Earth while blessing their own cardinals at the FSF Vatican and excommunicating any dissent. Given that kind of very overt signalling, it's just not defensible to argue any other primary target than the FSF and the overly central development process they were dependent on to maintain control over projects.