True, LE have never said a connection between the two cases can be ruled out. I have been re-listening to the Scene of the Crime podcast and in it Drew Collins, father of Elizabeth, is interviewed about his daughter's case and the similarities to Delphi. According to this podcast, LE DO know the cause of death for the Evansdale victims, though, as in the Delphi case, supposedly even the families have never been told what it is. Drew Collins says that he reached out to BP and MP in the early going to offer his sympathy as a parent who could relate to what they were going through and when he personally thought there might be a connection between the two cases. But, he says in the podcast, "nothing ever came of that."
Because of the long amount of time before the bodies were found in the Evansdale case, it makes it less likely that usable DNA exists (though LE has not said this for sure), so this makes it possible that there are other significant evidentiary differences between the two cases that are known to LE but not the public.
One thing that is known about serial child abduction killers from studies into their behavior is that if they use a particular ruse to abduct children in one case, they tend not to stray from that ruse in succeeding cases. And if they have a particular way that they kill their victims, they tend to stick to that method as well and not vary too much.
