The 2391 exam is a bit of a strange subject.
I will admit that I took it too early, I was offered it at the end of my CG parts I and II, and revised well and passed it. I have also known people I would regard as competant electricians fail it.
To a certain extent a basic grounding in electrical theory coupled with learning the topic along with revising it can get you through the exam, where you go from there is another matter... does that make one an electrician though? well not really.... it didn't me.
I was lucky, I got into the contracting sector as an improver, shadowed people for a bit, made daft mistakes that apprentices do, that people expected me not to because I was older...
Even now a good few years done the line, having done few years in the trade and got my JIB card etc, I'm more maintance/testing/design/fault finding based ..... I'm not the guy you'd choose to make off big SWAs or conduit a whole site out, quite simply because I'd be too slow at it, as its not something I do much of!