Stephen King answers basically with the same advice he's always given, as you said, he tries to write 6 pages a day.
In the video, he says he tries to get 3-4 hours of work a day, and get to that 6 pages. He breaks it down, "if the manuscript is x length, it will take y months to finish." George RR Martin goes on to ask essentially, "but what if writer's block? What if internet distraction? What if something comes up?"
He goes on to tell a story about Rowling that basically resolves with "nobody really understands what writers do," but based on King's own "On Writing," I think he does claim to understand, and his tips are not really any different from how you generate content in any other medium, methods that those of us terminally online HN types are at this point experts via headlines alone, we see so many posts on the subject.
The method is demonstrated in the "6 pages" quote that gets thrown around: define the objective (write a book), break the objective into estimable and achievable goals (write 6 pages a day: this allows for book length x to be finished in y months), and set dedicated, distraction-free time to achieve the day's goal (as I recall from "On Writing," King has a dedicated writing office and doesn't bring his phone into it, and doesn't have internet access in there. I may be remembering incorrectly or mixing up my absurd breadth of reading I've done on the general topic of staying focused).
Meanwhile GRRM, for all his undeniable success, talent, and skill, talks about the act of writing in kinda the same way an amateur might, or maybe just the way an artist might, a sort of mystical "waiting for the muse" type style. "What if I just don't wanna write?"
King's answer, and the answer of many prolific artists, programmers, etc, that I've read, would be "why are you whether you want to write or not? You have today's goal already, so, achieve it." The wanting is irrelevant, the muse is unneeded, you've broken down the task, go do today's TODO list. Are you going to re-justify for yourself every single day the overall objective? Well, tbh, I do like that as a mindfulness self-affirmation exercise sometimes, but the point is you can't just sit around waiting to want to do the small part of the larger objective, you gotta just put the hours in day after day.
I think a lot of us across all disciplines are missing that key skill in production: project management, I think I'd call it. The most prolific artists are the ones who grabbed that skill and added it to the toolset as a discipline-agnostic ability. Hence why when King talks about his method it sounds the same to me as what I've read here from programmers, what I've seen on youtube from animators, and what I've heard on podcasts from musicians.