However, a siege tank's splash will still affect a unit inside swarm unless it is burrowed. It seems that burrowing makes the unit size or hitbox a bit smaller or something, I'm not sure. That is why one lurker burrowed under swarm cannot be hit by a siege tank's splash.
However, since the attacks are moved down one matrix, a siege tank can still actually hit something in swarm for splash. So the siege tank targeting a lurker above a defiler's attack can hit the defiler with splash damage.