A patent has to disclose the invention in order to be valid. An invention, however, can comprise a new and non-obvious combination of pre-existing components. Most inventions, in fact, comprise new and non-obvious arrangements of familiar components, some of which may be patented by others. If a component is patented by another, that just means the inventor won’t be able to actually build his invention without buying or licensing the component from the other inventor. I’m not familiar with the Marconi case, but that may be the situation.