It's like, I don't know, a thousand times more likely that someone not in prison was scamming the system and one of the easiest ways to identify that some claims were fraudulent after the fact is to check them against databases of very ineligible people that might not have been checked in the approval filter. "Currently imprisoned people" sounds like one of the most likely lists, since long dead people are probably somewhat more likely to be filtered out on the first try.
"People currently in prison" is probably also a fairly easy list to hit since there are public records indicating who has been sentenced and the people in question are way, way less likely to be tipped off that a fraudulent claim has been made in their name since they are largely cut off from the world.
Fraud happened, and surely some people in prison were able to hear about the increased UI benefits and apply, but I have a very hard time believing the headline.