Unless you are only talking about rebounding, I would say this is a truly terrible post.
Obviously guards have to prevent penetration into the interior of the defense, whether it's man or zone.
They have to challenge shots and be aware of 3 point threats. They need to disrupt passes to the interior, or skip passes across the court.
Even if you were talking about guards not being important to defensive rebounding, I probably would still disagree, and think that a 6-6 shooting guard should get 3 or 4 rebounds a game.
In fact, he should be at least turning and looking for the long rebound before he takes off downcourt looking to spot up for three.