I own a Cellebrite, and yeah you are right. The Cellebrite box is nothing other than a phone backup tool. The nice thing it does is implement every backup sync protocol for every version of every mobile OS so you don't have to spend a whole day trying different combinations of iTunes and such.
The "Physical Analyzer" is just a forensics tool. There are dozens of competitors out there that will take a phone and surface the things that might be interesting in a court case or law enforcement investigation.
The product Signal didn't talk about - which I think is the one they are upset about - is Cellebrite Premium. That is their service where law enforcement can send locked or damaged devices to their lab and get back a an image to load into PE. However in 99% of cases devices are either accessed because they are running old software with public vulnerabilities, or using the magic phrase "would you mind unlocking your phone so we can clear this matter up?"