I don't agree with that. Bamber testified Sheila had 'limited' knowledge of guns. Probably 'limited' because he couldn't say 'no'.
JB would need to expand on "limited".
It did form part of the prosecution case against JB unlike much that is discussed on the forums:
b) Save for the appellant nobody had seen her use a gun and she had no interest in them. Sheila Caffell also had very poor co-ordination and would not have been capable of loading and operating the rifle nor would she have had the required knowledge to do so; Jurors were allowed to handle the rifle and magazine and I think load bullets, dummies I assume.
Personally I think its a perception thing in the UK. Most don't have access to firearms so its seen as alien and certainly not something a city dwelling attractive slim woman would ever handle! Case related debates from the US have a different take on this aspect of the case.
Surely it would be possible to take a bunch of novices allow them to observe an experienced handler load the mag, attach it and chamber a round to see how many times on average they need to observe before the task could be successfully completed. If most were able to master the task after observing a couple of times then I think we could safely assume it was straightforward.