It doesn't define aerobatic manoeuvres - it states what the Bell was designed for (50 deg AoB, 15 deg nose down and 30 deg nose up) and mentions the RAF (not RFM or any civil document) limits of 90 deg nose up and 90 deg AoB).
There is a lot of information about possible contributory causes of the structural failure including the teaching of wingovers (in the RAF syllabus for many years but apparently not picked up by Bell until they were told) and sloping ground landings - specifically the technique of reducing an out of limits slope in one direction by adjusting the heading to make it a compound slope. This has been taught for many years on many types in the military and has never been questioned since you remain inside the limits for nose up and lateral slopes.
Bell decided not to replace the support structure for the MRGB post the wingover revalation - it says this in the report - so they can't have been that bothered by it - perhaps a combination of the stresses of repeated sloping ground landings and repeated wingovers could have been modelled to suggest a different cpourse of action.
In the end, if you land out of limits on a slope (even using compound techniques) and use an inappropriate amount of forward cyclic, you are putting a lot of strain on that support structure which appears to have given way.