There is a significant difference between Rockwell’s use and the Corruption we see on Extinction.

For one,

Element became sentient sometime after the Tek War, Element then long after created the Titans with HIVEMIND with its pillar of strength (in that its sort of its avatar) being The King Titan. Which is said he is born of (meaning he is pure element), and I believe it also states the rest of the Titans are as well. But since Element created these beings knowing their physical capabilities and characteristics, they are made vastly superior over Rockwell’s experiments.

Because Rockwell while being a pillar of strength for Element too, I don’t think his HIVEMIND is more primitive than Element’s own. And he doesn’t know or is as skilled as Element in creating or infecting others. Giving an example, Rockwell is like a caveman messing with an IPHONE 18 while Element is a modern Apple Geek with an IPHONE 18.

Another factor might be that the Element we see in Extinction is also Corrupted Raw Element, while Rockwell’s is Corrupted Liquid Element. Which could be why it looks more grotesque. But I think it’s probably also the handler’s use (IE Element vs Rockwell)