Using that same logic: since you don’t know whether victims of unsolved murders knew the killer, how could you know which is easier to solve?

The person you are replying to is making a rational argument, not an empirical one. The premise that may be understated is "cases are more likely to be solved when the victim is known to the killer, because the amount of people who know a person is far smaller and more easily investigable than the entire general public."

Oh, I understand the premise. But one premise is not a good argument. Maybe witnesses are less a likely to cooperate if they know the murderer! Maybe this type of murder tends to happen inside homes, where there’s less likely to be evidence such as surveillance footage! Maybe it is easier but only by one or two percentage points! etc etc