Agent Skills isn't a spec for how information is presented to the model, its a spec whose consumer is the model harness, which might present information made available to it in the format to the model in different ways for different harnesses, or even in the same harness for different models or tasks, considering things like the number and size of the skill(s) available, the size of the model context, the purpose of the harness (is it for a narrow purpose agent where some of the skills are central to that purpose?), and user preference settings.
The site itself has two different main styles of integration for harnesses described ("tool based" and "filesystem based"), but those are more of a starting point for implementers that an exhaustive listing.
The idea is that skill authors don't need to know or care how the harness is presenting the information to the model.