descendantOf() parameter, entryModel or ID?
These are two different things.

{% set entries = myEntryModel.myEntriesField.descendantOf(var) %}


In this case the Entries field returns an ElementCriteriaModel object and you are setting the descendantOf parameter, which does in fact accept EntryModel objects or an Entry ID as an integer value. The Craft documentation is incomplete in this regard.

{% set bool = myEntryModel.isDescendantOf(var) %}


Here you are using the method isDescendantOf that is available for an EntryModel and returns whether the entry is a descendant of another entry. This method apparently only accepts an EntryModel.