Pulling this across from the other thread (which is the IM Pro Series 2025 thread, IM Rules is peripheral (but Rules wef first IM Pro Series race (deliberately). Will Skipper have a final bottle sing-song in Taupo?
I’m going to assume the rules drafter knows the intent but maybe struggles with language. The thrust is obvious but the intent is to head off (proscribe) bottles attached to occupy the space under the rider’s chest. So while mechanically the top tube (and indeed the head tube or the frame itself) cannot be described as a "rotating around the steering axis” the drafter has lazily lumped them in. A better draft might read:
"Systems mounted to components attached to the front part of the bike (i.e.: top tube, head tube, headset and those that rotate around the steering axis (bars, cockpit, stem and fork)) must have a combined volume capacity of no more than two (2) liters”
(Attaching stuff to the downtube inside the triangle is OK’d in a subsequent sub-sub-paragraph.)