There's not that much vagueness, either a bullet is jacketed or it isn't, if it isn't, it can't be made out of one of the restricted materials and be legal for import, if it is, and it is larger than .22, it cannot have a jacket greater than 25% of the projectile weight, and the core cannot be entirely composed of one or more of the restricted materials.
There is nothing wrong with having a multiple material core, as long as at least one of the materials is not listed in the restricted materials list.
These are all known terms, there is no confusion about that copper-jacketed, lead and steel cored bullet higher up the page.
M855/SS109 does not have a steel core either, it has a lead core with a small piece of steel in it. The core has two different metal components, the one that could cause import/manufacturer headaches is not the sole component and in the case of SS109 it isn't even a large amount of the core.
It's not vague. The core can be made of two or more different metals, blended or not, but as long as the core is not entirely composed of metals listed on the restricted list, there's no issue with it.