Ok, so let's break down this a bit: first, yes, he did shatter Mannoroth's weapon, but here's the thing: it wasn't his pure strength either. The strength of the impact was also increased by Grom's momentum as he fell down from his jump. Two, it's a downward swing, and considering the armor plate on Mannoroth's chest is loose and just being by chains from his shoulders, it's also valid to say that Grom's blow hit over the plate. We don't actually see the belly plate being shattered, just the weapon.
On top of that, I imagine the Lich King is a much smaller target than Mannoroth, so it's harder to hit than a mountain of a demon. The Lich King can also rip a piece of Grom's spirit to fight against him.
DK Arthas vs Fel Grom would be an interesting match to watch. Lich King Arthas vs Fel Grom would just be the LK moping the floor with Grom.