This is a very confusing aspect of Inform syntax.
The narrow sense of “visible” seen in action definitions such as Considering is an action applying to one visible thing. is actually synonymous with “in scope”, rather than the sense of visible defined by the visible adjective, i.e. something which can be seen. Objects placed in scope using the ‘deciding the scope of’ activity become forcibly “in scope” (and so “visible” in the first sense, i.e. they can be referred to in a command in order to generate an action) but they are not at the same time necessarily made “visible” to the player/actor in the latter sense- i.e. if the player/actor couldn’t see them before, then they still can’t necessarily see them- they will only be made “visible” in the latter sense if the player/actor is not in the dark.
For more information on the meaning of scope, see here.