Have you ever seen the plot device in fiction that goes "in this other universe, X happened instead of Y" or "A exists instead of B"? Like, the Axis won WW2 instead of the Allies, or the Earth is a cube rather than a sphere?
When Holomanga here is talking about an internal perspective for cosmological subjectivity, he refers to a setting in which "in this other [cosmology], A exists instead of B". In order to accommodate for all of these happening at once, you have to sort of ignore the contradictions they'd cause (e.g. one cosmology says only one universe exists, another cosmology says multiple universes exist) and just say that you can access any of those cosmologies from within your fictional setting.
Not that you actually have to do this when you're writing; this is really just a formalism we apply on the wiki to keep everyone included, and when you write a story or something it's easier just to use the bits of peoples' cosmologies that you need.