Someone could check me on this, but I think it wasn’t about her literally becoming a queen. I believe Kurtz said something about her becoming the leader of whatever is left of her people. I suppose that could be interpreted as her becoming Queen to any survivors of Alderaan, but I always interpreted it as her becoming the leader of whatever is left of the Alliance at the end of the movie. Since she is Princess Leia, I figured people misinterpreted that as her becoming Queen.
A quote from Gary Kurtz on that subject (in bold):-
Q: “Let’s jump right in. I have to ask, what exactly were your initial thoughts about the third Star Wars film, which was originally called Revenge” of the Jedi?”
A: (Gary Kurtz) “The one story thread that got totally tossed out the window, which was really pretty important I think, was the one of Vader trying to convince Luke to join him to overthrow the Emperor. That together they had enough power that they could do that, and it wasn’t him saying I want to take over the world and be the evil leader, it was that transition. It was Vader saying, “I’m looking again at what I’ve done and where my life has gone and who I’ve served and, very much in the Samurai tradition, and saying if I can join forces with my son, who is just as strong as I am, that maybe we can make some amends.” So there was all of that going on in Jedi as well, that was supposed to go on. So the story was quite a bit more poignant and the ending was the coronation of Leia as the queen of what was left of her people, to take over the royal symbol. That meant she was then isolated from all of the rest and Luke went off then by himself. It was basically a kind of bittersweet ending. She’s not his sister that dropped in to wrap up everything neatly. His sister was someone else way over on the other side of the galaxy and she wasn’t going to show up until the next episode.”
^ From the original ‘Original Star Wars Producer Gary Kurtz speaks’ article in 2000, at FilmThreat - https://filmthreat.com/interviews/gary-kurtz-interview-the-original-star-wars-producer-speaks
There may be other quotes from him along similar lines in other articles and interviews etc too? (though that is the main one I can recall)