What the report actually states about the camera evidence:
The images included a part of the ASI, some of the fuel system controls and
indicators, some of the oxygen system controls and indicators, the JPT indicator,
and occasionally the engine rpm gauge when the pilot moved sufficiently for it
to come into view21. Determining their indications was complicated by image
resolution, contrast and partial obscuration.
The altimeters, thrust levers, and many other indicators and controls, were not in the camera’s field of view.
Why "would have"? We were told that the GoPro could see the altimeter. The report might have said "should have" or just "indicated". We are not told what the conditional tense is for. The report does state that the GoPro had poor performance when inverted:
Can you reference the section of the report that comes from?
Perhaps you are getting confused with this, which in no way suggests poor performance on the part of a GoPro when inverted. The reduced value of the video comes from ambient light.
1.11.7.4 Recent Jet Provost display flying
The pilot displayed a Jet Provost on both days of the weekend prior to the
accident. Cockpit video recordings from both displays were provided. They
captured the airspeed indicator and altimeter on both days but also suffered
the same image contrast problems as the G-BXFI cockpit imagery when the
aircraft was inverted. Two loops were flown as part of the display on both
days. The indicated airspeeds and altitudes for each of the loops are given
in Table 6. The wider tolerance of indicated altitude in the 'loop apex' column
accounts for the reduced quality of the video due to poor image contrast when
inverted.