It's not a Huntress ability. He means the Vorpal Blades ability used by Glaive Throwers.
As the description of Vorpal Blades sais:
Permits the glaives launched by the Glaive Thrower to strike through initial ground targets and damage those behind. Also allows the Glaive Thrower to attack trees.
You do however need to edit that it only allows to attack trees. And since there is nothing in the Object Editor or Vorpal Blades that can edit that, and there's no sign that shows it's possible to attack trees through a part in the Object Editor, I think you can only add trees to possible targets of a unit by using a JASS code. It's probably just 1 line, but I've never used JASS before and know absolutely nothing about it, so I think you should ask someone to make that JASS code for you.