There is only one 30-seater bus coming from Tsing Yi. Many of these kids come from either Ma Wan or Tsing Yi. Very few come from beyond there. All the other buses come from Lantau, either Tung Chung or Lantau South. I don't think the catchment of Discovery College is any bigger than any of the other regular ESF schools. The remoteness of the school self-regulates its catchment.
Many of the regular ESF schools are oversubscribed. So even if one falls within the catchment, they're not guaranteed a place. I don't think Discovery Bay kids experience anything different than kids in other areas of Hong Kong. There's simple a shortage of desired school places.