Is the swarm guardian the same thing as a regular guardian? If Zerg has no ranged siege unit how will it compete?
The Swarm Guardian is similar to the original Guardian, acting as a flying long range siege unit. In addition, the attacks also spawn Broodlings upon hitting the target, which do additional damage to the enemy, lasting only a few seconds.

Cool? It just sounds like Poison with 'broodlings gnawing' visual effects. It's not bad but uh.
To clarify, the Guardian's attack does not add a DoT (Damage over time) on the targeted unit. The targeted unit will take damage from the attack and then two Broodlings will spawn adjacent to that targeted unit every time the Guardian attacks. Each Broodling acts as an independent unit, controllable, and killable. Over a few seconds, the Broodlings will die on their own. With just a few Swarm Guardians, you can create an army of Broodlings upon your enemy in mere seconds. Nonetheless, these Broodlings do little damage and have little hit points - but with enough, it will definitely force your enemy to retreat.

So I guess Broodlings won´t spawn if you attack buildings?
They spawn when you attack buildings as well.