I find this and the previous example confusing regarding the need for "nawzajem";
oni się rozumieją nawzajem
oni się lubią
„Nawzajem” means "each other", but if the context is known, it can be dropped. Without „nawzajem” this sentence can be understood in two ways: "they like each other" (A likes B and B likes A) or "they like themselves" (A likes A and B likes B).
My first idea was They like themselves.
Which actually works as it is a correct interpretation.
“They like themselves” was not accepted in my case, but there seems to be a clear difference in meaning between that rendition and “they like each other”.