We must start a protest with signs saying "GRANDFATHERS ARE PEOPLE TOO!!!"
But jairapetyan isn't talking about grandparents of mixed gender, just about grandparents. We don't know the context of this sentence; it might be spoken a few years after a disaster wiped out all adults of one gender. Then there would be no parents of mixed gender at all, and saying "geavoj" would be completely wrong.
What jairapetyan needs is the plural of grandparent in general, grandparent of unmentioned gender. And since Esperanto doesn't distinguish between male and general, that word is "avo", and the plural is "avoj". Thus, "Avoj estas homoj." (no accusative with estas), does indeed also mean Grandparents are people.", even if it wasn't what the constructors of the course intended.
Cent tridek jaroj senprogreso.