My best guess would be that it might specifically apply to planeshift/gate type magic but that places within the world that allow creatures to travel to planes such as the feywild might slip through?

Well I wondered if the Feywild being part of the Material Plane was the reason why it was still allowed but the Elemental Planes are also part of the Material, right? So that doesn't really make sense. And does the Tree of Names affect the Feywild in the same way it does Exandria? Would the Fey have allowed that? And if it doesn't protect it what would stop someone from planeshifting into the Feywild (or Shadowfell) as an intermediate before gaining access to Exandria?

Obviously the Doylist explanation is most likely Brennan/Mercer didn't consider this aspect but I wondered if there was something I was missing.