I've been using a relativistic energy calculator: https://www.omnicalculator.com/physics/relativistic-ke
The baseball in that scenario would have about 4030922 tons of TNT worth of KE.
EDIT: I WAS LIED TO. Yeah, that does actually end up within the strength of a nuclear bomb. They were probably about right, though I'm still not sure why they focused on fusion and said nothing of the KE. They might be tied together, in a sense.
@The Bookwyrm