Creating the story itself isn’t necessarily any different or easier simply because we know who Luke Skywalker is.
Even though Star Wars: The Last Jedi was part of a trilogy, that might have included characters that we know, the movie itself was still a brand new creation that would take those characters to new places.
Rian Johnson does have a point.
That’s not to say that the story wasn’t in some way shaped by those existing characters, but at the end of the day every script has to start from scratch and you’re certainly not guaranteed that a sequel will be better simply because the characters are known by the audience.