I agree.

If the viewer didn't know it was a true story, it would be very hard to believe that an undercover agent, in 1975, could just walk into a social club on Mulberry Street and have a guy like Lefty approach him about fencing diamonds.

Had they invested some screen time with Jilly and his crew, the average viewer would then find out that a made guy (Jilly) had vouched for "Donnie" BEFORE he did any actual business with Lefty.