If a product is alleged to have feature X

Alleged? By whom? When? On what basis? And whom alleges that someone alleged that the product had this feature?

The problem with not intelligently differentiating between missing features and bugs, is that it makes it impossible to prioritise properly.

A bug is (should be) a higher priority than a missing feature. It is something that prevents an implemented feature from being useful until it is fixed.

A missing feature is lower priority because if the bugs in the implemented code are fixed, a less featured product may still be useful and shippable.

And that pretty much addresses your "dependant feature" argument. If an implemented feature doesn't work without an unimplemented feature, then both are miss-categorised. The dependent isn't complete, so is not yet an implemented feature. And the dependency is not a feature, but an unimplemented subset of the dependant feature's functionality.