When testing the pressurisation functions of an aircraft, or checking for door seal leaks etc., it is OK to sit in the flight deck and monitor using the aircraft gauges. However, after manufacture or a major repair, the aircraft is more often pressurised from outside the aircraft and the internal pressure monitored using an external gauge. This is not an AMM procedure, it is part of an approved repair scheme.
The information provided above says that the aircraft had just undergone a Major Check and it is quite likely that the aircraft was being pressurised and monitored externally. Many years ago while I was still in the military, a colleague of mine was checking a repair for leaks and as he approached the aircraft, the repaired structure failed and he was killed. He had not been involved in the repair either...
RIP, Steve old friend. Another hard lesson in engineering that I've never forgotten.