Yes, that is kind of non-sensical as written. I'm going to guess that by "antenna" they are thinking of "resonant dipole" and by referring to a trombone, they

Yes, that is kind of non-sensical as written. I'm going to guess that by "antenna" they are thinking of "resonant dipole" and by referring to a trombone, they are saying the grape is more of a resonant cavity. But then again, a trombone is not a resonant cavity in the RF sense -- it resonates as a tube closed at one end.