Yes and no. You need retinues to beat stacks MUCH larger than you but just knowing how combat works can beat AI stacks larger than you as long as it's not overwhelming odds.

Even if you have the LoR DLC, knowing how combat works can in CK2 can still help against larger armies in situations when you can't even really use retinues yet. Starting as Harold Godwinson (much different than CK3's start), Thankmar in 936, immediately attacking Alfonso as Sancho in 1066, immediately attacking south as Sigurdr in 867, etc.

I'm not talking about just good movement/favorable terrain either which obviously helps, but winning actual battles against larger armies with just levy compositions by just reorganizing them a bit, finding the right commanders for the composition you have, or buying mercs with certain compositions.