I'm not sure why a patent application for the design & construction of a lens has to have every single design point included. The IS part of a lens is already covered by patents, and new patents have to cover something new & original.
There is another point to consider - Canon offer 2 different 70 - 200mm lenses, and offer them in IS & non IS versions. There is nothing here to show that Canon can not / will not offer similar in the 24 - 70mm f/2.8 satisfying budgets and market demand.
The is unit contains optics and thus is part of the optical path being patented. It might not be identified on the patent as a IS, but it needs to be included in the optics.