While the first part of your post was correct (knock is vectored from the caster’s location which is not necessarily the center of the power doing the knock), the part I quoted isn’t strictly true.
Why? Well, you see, neither of those powers are guaranteed to knock mobs. What’s more, Energy Torrent has a 60% chance while Explosive Blast is only 50%. What this means is even if you’ve got mobs up against a wall or whatever you’re only going to send some of them flying. You can mitigate this by using fly or teleport and only ever using those powers directly above the mobs you want to knock, but this isn’t at all practical on indoor maps.