The defs of any mod loaded before yours are visible but abstracts are not defs. Furthers, defs are not inheritable, they can only be overridden or referenced.
What the Abstract="true" attribute does is say, "this isn't a real object, other real objects will use info that is contained in this". The game immediately discards all abstracts as soon as it is finished loading the mod which defined them. ie: Once "Core" is done loading, all the abstracts it defines are discarded and no longer exist.