During the earlier part of my writing career, I wrote three novels based on my life and times in Chicago. I was taking that good bit of advice: "Write what you know," and my life was in fact something I knew. I expanded on those stories, creating fictitious characters to conceal the identities of those who really didn't deserve to have their most embarrassing moments published by me, and even making up a bunch of wild plots barely related to my actual life. It was all in good fun and taught me a lot about writing, but was I really following that advice of, "Write what you know"?
Think about this. How much about space travel did Arthur C. Clarke know when he wrote 2001: A Space Odyssey? How much apocalypse experience did Stephen King have when he wrote The Stand? When H. G. Wells wrote The Time Machine, I am guessing his experience with time travel was limited, to say the least. However, these are fine novels somehow created without using the advice about writing what they know. Every bit of sci-fi, high fantasy, and alternative history fiction was created without using that advice, right? Well, they actually did follow that advice; just not in the conventional way.
Thinking about 2001, this is not a book about space travel. Spaceships were involved, but Clarke was writing about different themes: The Cold War, concerns about technological advancements, and artificial intelligence. Clarke was well versed in these subjects and discussed them often, and eventually synthesized what he knew with other ideas and previous stories to create this masterwork. He wrote about subjects he knew, not events.
Of course, The Stand follows a similar approach. Stephen King had written just four books and experienced zero world-ending diseases when he wrote this epic novel. What he did know, however, is how to get in touch with fear, with horror. He knew what scared him, and what scared everyone. He got in touch with the things that kept him awake at night, and wrote about those. The apocalypse was just an incidental part of telling a story touching upon very real subjects.
When we write about what we know, we need to think about just what that means. The first step can be writing about events. However, more readers will connect with the author that writes about feelings, fears, and themes they can relate to. That's something we all know.