I get reed-stem Epi "volunteers" in other pots (mostly Catts) fairly often. So I suspect that they are a lot less picky about getting the perfect mycorrhizal environment than most other orchids.
Some reed-stem epis are an exception to the general requirement for orchids to have a mycorrhizal associate to germinate. The seeds are a bit larger and do have some endosperm. Genus Disa is another exception.