MDF is what the stuff usually gets called around here. Masonite seems to be the subject of a lot of confusion- I've heard it used in reference to drywall a few times.
Home Depot in general doesn't staff very well in my experience. The one in my area is always chronically understaffed, and when you can get a hold of somebody, they usually aren't very knowledgeable. Twice I've actually been lied to because (I suspect) the salesman didn't want to admit not knowing. I think they just just don't bother to screen or train their hires very much.
There's an Ace hardware a couple miles further away from me, and I've started to go there instead. Smaller location, but better selection and better staff.