BUT there is some conflicting info on this, so think we may have to see on this. Like a lot of Genie, it is perhaps still in flux.
From the guest facing Genie site:
When to Select Attractions
You can purchase individual selections for up to 2 different attractions per day.
- Guests of a Disney Resort Hotel and Other Select Hotels: These Guests can make their first purchase starting at 7:00 AM on the day of their visit. Available to Disney Resort hotel Guests and Guests of Walt Disney World Swan and Dolphin Hotels and Shades of Green.
- All Other Guests: These Guests can make their first purchase when the park opens.
And in another place:
Lightning Lane entry as an "á la carte" purchase will be offered at a few of our most popular attractions. This option can be purchased by all Guests—with or without the purchase of Disney Genie+ service—and lets you choose a time to arrive at the Lightning Lane entry at up to 2 attractions each day (purchased one at a time).