Yes, that might be the case under the current circumstances. The above picture helped to imagine what 500 people look like crammed squarely in a space. I doubt the 500 hypothetical people resting in the parking lot were placed more tightly than that.
Next, look at pictures of the detonation crater, which has the size of a sewer manhole. Imagine that explosion going off in the middle of the audience of 500 people. Would they all die from that? The sheer amount of body mass and flesh would shield the people in rows 20+ from the explosion.
I bet you couldn’t kill 500 people with whatever exploded there, if you tied them directly onto the bomb, or rocket, or whatever. Is there any precedent of a terrorist attack or military strike or anything which killed that many people with a similarly small explosion?