Correct. Even with perfect conversion of mass to energy, it would still take converting the mass equivalent of a 16+ km asteroid directly into energy.
Even antimatter explosions are not anywhere near 100% efficient. They produce many subatomic particles with high energy. Much of the reaction will quickly (on the order of 1x10^-16 seconds (... ok ok pions take 1x10^-8 seconds to decay, which is much longer)) settle into neutrinos or electrons. About half of the energy would nigh-instantly convert to neutrinos. Neutrinos will not care or contribute to ripping Earth apart even if they're produced in the Earth's core.