Where do these people get their information?
I always try to get my sources directly from Wikipedia whenever possible, because I know everyone has free access to that, but I also use various other free internet sources. If all else fails, I'll source from books, but it's as a last resort, since I'd rather not make people pay money, or take a lot of time, to verify things.
Other than that, my dad actually made his living, for several years, as an antique sword appraiser for auction houses in Pennsylvania. Any time someone would bring in a sword for sale, he was consulted, as the expert advisor for the area we lived in.
So, from an extremely early age, I knew how a sword was put together, what the different kinds were, where they came from, and the manufacturing differences between a "parade sword" and a killing weapon (I knew that many years before I had any concept of death, itself, actually), how one actually felt in my hand and how they were used and cared for, and some of the most famous makers of swords.
Public school really couldn't compare to any of that, so it also taught me that I would always be my best teacher.
He also studied martial arts extensively, and collected rare books and other artifacts (and I would always go with him), so I grew up, in some real sense, in a museum, although it wasn't a cold or stifling environment--it was really a boy's paradise.
My grandparents also helped raise me, because both of my parents always worked, and they lived on an old farmhouse on a wooded estate, filled with antique tools that I was free to play with, so in my embarassment-of-riches
second boy's paradise, I learned about tools and using my hands, and about being out in nature, and spent a lot of time making my own fun, since I honestly didn't have many friends close to my age.
Once I could read, my favorite book growing up was D'Aulaire's Norse Mythology (although my dad is also a Tolkien fan), and when I was 6 years old, my mother arranged a play-date with an older boy from her work, and he introduced me to Dungeons & Dragons. From that I became a fantasy otaku, and the rest was pretty much history.
So 36 years now, spent just being around and studying history--particularly the history of pre-industrial warfare and martial arts, 30 years studying fantasy and mythology, too many thousands of hours spent playing games, and atleast a dozen years with access to the practically unlimited sources of the Internet.
None of that automatically makes me right about anything, but I'm usually pretty good at tracking down information, comparing it to what I do know, and then "translating" it into something that fits the framework of a game.