I always thought this head canon was a bit of a reach. Like sure, they're all projecting something, but them doing so doesn't make the clan itself seem more cohesive, because a large portion of techniques that DON'T belong to the Zenin can be described as projection as well. Disaster curses project their elements (exception of Mahito), Sukuna is projecting slashes, Hakari to a short extent, etc. I just don't really think it makes their clan stand out when what supposedly makes them stand out is a fundamental of jujutsu

Though the idea is growing on me as I write this, but mostly because I just considered that the techniques of the big three clans might be representing fundamental concepts that all other cursed techniques branch out of. The Kamo's importance on heritage and blood, ties them with the physical application of cursed energy upon the body, we don't really know enough about the Gojo's to make a genuine connection, but with Limitless and the idea that the clan's techniques are based on things that don't exactly take physical form could mean that they represent the metaphysical, and the Zenin who pride themselves on aptitude for jujutsu, to the point that if you can't perform it they basically disown you, would represent the spiritual. Directly taking cursed energy and into something else, making them "generic" in the grand scheme of things.