There are in fact millions of people who use Cash App, though it's probably less popular overall than Venmo. I know this because my wife and her friends will mention it and Venmo when talking about splitting bills. (Given the technical challenges Wells Fargo has had lately, I'm not sure who trusts Zelle.)
A lot of the claims amount to criticisms of Block's address verification functionality. But that doesn't mean Block engaged in fraud, perhaps they just have crappy address verification tech. Short of sending a human investigator out to physically verify that a given person really is located at a given address, I'm not sure what foolproof methods people use to eliminate that kind of fraud. The "let's send you a postcard with a code" method I've seen from legacy banks seems quite easy to exploit as well, just a bit slower.
The last question about merchant fees isn't fraud at all, but regulatory avoidance. Maybe the government will shut this down, or maybe they've found a loophole that Congress may want to close. It's unclear.
The least persuasive part of the report was the implication that rap lyrics imply Cash App facilitates crime.