I see the logic like this. Take it for granted that they’ll never end up releasing anything titled “Episode 10” ever.
If they’d never made 7, 8, and 9, and all the Legends/EU novels were films instead, you could consider the Thrawn trilogy to be 7, 8, and 9. Anything that substantively advances the Skywalker story at the time of its release (it is set further ahead than anything else already reckoned as an “Episode”), would be considered the next “Episode.”
By this logic, the movies would have been:
Episode 7: Heir to the Empire
Episode 8: Dark Force Rising
Episode 9: The Last Command
Episode 10: Jedi Search
Episode 11: Dark Apprentice
Episode 12: Champions of the Force
Etcetera. But Truce at Bakura, or Courtship of Princess Leia? No dice, as at their release they were set prior to where we’d gotten up to.
Presumably they won’t make nearly as many movies about the Skywalkers in this way as they published novels.