In this case I'm saying that we're over-abstracting by pulling too much of the dissimilar code between the examples into the one representation (i.e. the method).

Ok it sounds like we agree on what is wrong with the example. "Over-abstract" still feels like the wrong word for that, because the actual problem is that we have ruined our abstraction layer with junk about lower layers. Average is an operation on lists of numbers, but ignore_nulls is a feature of the members of the list in your programming language, same with the Type argument. The members of data structures intuitively (maybe not always) exist at a lower level of abstraction. I would be more inclined to call this something similar to "partial-abstraction", because the programmer didn't take the time to remove all the semantics of the lower levels from the interface to the higher level.