Making a new game engine doesn't take 11 years. The are individual people who have made modern game engines in a fraction of that time.
What comprises a "game engine" is very vague and is only ever defined as the engine is being developed, but it's rarely what makes games take a long time to make. An engine is usually just replicable game code, a lot of which can be accessed in the public domain and implemented straightforwardly. I'm not suggesting that it's easy, but that it takes a fraction of the manhours that the rest of the game takes.
When studios say "we are creating a new engine", it's just PR speak to hype up customers. Most of the time when they say this, huge amounts of code get copied over into the "new" engine. It's primarily an in-office thing that doesn't affect the end product.
For example I use unreal engine 4, and unreal engine 5 is just a small update from 4. They haven't really added much besides bugfixes in the past 7 or so years, but based on their PR statements it sounds like they spent years behind the scenes building this amazing new piece of software. It's the same with bannerlord. The monent you could see screenshots back in 2013, the engine was basically already done.