This is the rule.
He clearly lands on him with most of his weight. (#1). He hits him in the head. (#2). He had to dive onto Zach on the ground which means he didn't make an attempt to avoid the hit. (#3). Zach is out of the play at the time of the hit. (#4).
Article 5 — Roughing The Passer
Because the act of passing puts the passer in a particularly vulnerable position to injury, special rules against roughing the passer apply. Once the ball is released, defensive players must avoid all unnecessary contact with the passer. A player shall be penalized for any act of Unnecessary Roughness to the passer, including but not limited to:
Contacting the passer in an unnecessary manner, including stuffing him to the ground, violently throwing him to the ground, and landing on him with most of the defender's weight,
Any blow above the passer's shoulder,
All rushing defenders must attempt to avoid forcibly hitting a passer in the pocket, at or below the knees, either if their path to the passer was unrestricted, or if they are coming off a blocker,
Attacking the passer who, after releasing the ball, is either standing still or fading backwards and is obviously out of the play and remains out of the play,
When the quarterback slides feet first, all unnecessary contact must be avoided. The slide must be done in a timely manner to allow the defence to avoid such contact.
Contacting the passer if either the initial source of contact, or primary source of contact, is the defender's helmet.