Many people who played with Dave Arneson said he had a lot of the “rules” for Blackmoor in his head. It wasn’t written down. The lore of D&D’s early creation was that Dave sent Gary eighteen pages of hand written notes which represented the “rules” of Dave’s Blackmoor game. By today’s standards, OD&D has a very low page count but still way more material than Dave handed Gary in that initial stage.
I’m speculating, but it could be that the open system might have developed as result of Dave simply not writing things down and codifying them. As D&D has changed over the last 45 years, there has been this inclination to add more rules. Designers and DM’s have answered the clamor from players for more closed systems with more closed systems. Players wanted defined powers. You can do X, Y and Z. The designers designed. It occurs to me that the inclination toward more closed systems was incorrect.
Dave, because he had refereed war games for years and years, didn’t need a set of written rules in front of him. He had a good understanding of what the dice would do. He had a knowledge of heroic fantasy fiction and film. That was enough to get started with Blackmoor. My suspicion is that rules only were written down as patterns of player behavior emerged. The rules emerged from the interaction between the structures Dave put in place at the beginning of Blackmoor and the players as they went through adventures in Blackmoor. Dave only wrote down what he needed to write down.
What Dave wrote down was the foundation of the closed systems of D&D but because Dave had experiences as a referee, he was able to roll without a closed system (rules that had hard and fast interpretations) and this allowed the open system to exist. He was comfortable with saying, “Roll a D6 and if it comes up 1 or 2, there’s an alchemist in town who can make poison.”
He didn’t need to look it up, he just made it up.