I checked the CM internal website, here's a direct copy/paste from that site
Both the morning and evening Extra Magic Hours will be available to Guests staying at Disney-owned and operated resorts, as well as Guests staying at the Walt Disney World Swan and Dolphin hotels, Shades of Green and the Lake Buena Vista Hilton.
SOG wasn't included in May because they chose not to participate. Resorts outside of Disney were asked if they wanted to participate by paying a fee. SOG declined evening ones and only accepted morning ones back then. I'm sure they had many suggestions from their customers and they could now be participating.
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