I'm not sure what kind of 'fact' you are talking about. Before hexes, there was tabletop wargaming and it didn't have squares of hexes, it relied on using measuring tape. And that's precisely the background Gary Gygax had, so he naturally expected that other players would have it, too.The fact is, there was no such thing as a "grid" or a "battlemat"...There were hexes.
I'm not an expert by any means, but from what I've heard, for Gary D&D was simply an extension of small-scale, squad-based wargaming. Dave Arneson was the one injecting a healthy dose of roleplaying to turn it into what we think of today, when we talk about RPGs.
Back when we played AD&D 1e and BECMI D&D, we quickly turned to using graph paper and tokens or minis to represent combat situations, because without any kind of representation, combat encounters quickly turned into heated arguments.