That's because you're not looking at the
Task object. You're looking at the Activity History object.
From that documentation, the field name is
isTask. I'd imagine that the link to the actual
Task is stored in the
The Task and Event records don't need an
IsTask field, because we know what they are. The
OpenActivity objects contain both tasks and events, and are pseudo-objects, as they derive their values from two different tables. In normal database nomenclature, we'd call them "views", because they represent an underlying query of different objects.