I highly doubt any airline safety manual suggests cabin crew should be lifting heavy objects that they have not be trained properly for.
If a cabin crew suffered a back injury due to the lift, and the arrogance of a singular passenger who has potentially packed 20kg in their carry on, the whole flight will be delayed. This would cause untold disruption to 500 passengers...
If an airline puts this requirement in their manual, their legal department would require that training is provided. If not the airline would be liable for every cabin crew injuring themselves doing this....
Cabin crew of course can open of close lockers. They should not be lifting anything but their own belongings in or out. If they do it's at their own risk and a bonus. Should not be expected