You have a good point- we have seen varying levels of spirit-created matter that affects the normal world entities to varying degrees. People without a link to spectral energy can't see Muses ink, but they can see Isaacs weather manipulation and Isabels paper constructs, and interact with them.

With the barrier only affecting spectrals, it's most likely the same 'level' of construct as Muses' ink.
With the supplied context (thank you for diving through the archive so reliably, btw), that's about as good as I think I'm going to be able to guess without further advancement in the story.

It's fascinating trying to piece together how the rules of this world work, isn't it?