I think that a couple things are going on with the script there. Within the scene, they're running up the emotional ladder from Anthony's electric car to "Was it a boy?", so they can't get bogged down on Fredo. They have to keep his whereabouts vague, because his big reveal has to come later.

Within the movie, the scene sets up Kay hurting Michael by specifically gloating that she aborted his son. That doesn't really resonate unless they establish that Michael's irrational on that specific point.