SpyPilot says: "I find it peculiar that there are abundant details in the letter covering the period between 7:24 and 8:14 but no explanation at all as to why they allege that the aircraft was without any communication at all for 91 minutes."
The comm at 6:45 CDT was probably NW188 checking into that sector, it was prior to the point where they became deaf to further comm's and blind to ACARS and without interruption of further ATC calls until the attempted hand-off at 7:24 CDT. This would be the first indication to ATC that all was not well, and therefore is properly the focus of attention. 91 minutes between transmissions from NW188, and 50 minutes of it after ATC had reason to figure out what was up.