D&D is the rules system. You can bolt on your own setting, including a hyper-realistic medieval world. I GM'd that game, or its derivatives, for almost 26 years, and never really used their (frankly terrible) generic fantasy setting, since the fun part for me was coming up with our own world. There's nothing mechanically that prevents you from running a medieval game with D&D rules. There are better systems for it, but there's nothing stopping you. You just don't want to.

I don't blame you for that. I don't want to run one either. Which may point us to why D&D is ahistorical: a realistic medieval game would be of limited interest to most people.