There is a difference between running in the background and being in memory.

Recent Apps may or may not be in memory, that’s why when you switch to a “recent” app not used in a while it restarts anyway instead of continuing where you left it, it’s effectively just a bookmark to a closed app. So the list being long doesn’t mean these apps are using memory.

While running in the background is unrelated, apps can run in the background regardless if they are in the Recents list or not. And it can absolutely cause issues or excessive battery usage, but clearing the Recents list is not the solution.