Blippy, instead of making the sweeping generalisation "in other training environments and jobs it is possible to obtain leave when you need to" how about providing an example? In particular one involving group training from a similar type of job to ATC.
Of course it's possible to know 100% of a limited set of knowledge. To be (ever so slightly) facile, you know your alphabet I presume? You even agree "ATC's must know their separation standards" - pretty hard to achieve that without knowing them 100%. As I said, anything less than 100% of a standard is no standard at all.
I'd argue ATC is not a perfectionist culture - knowing the rules is only one part of the job. A bigger part is applying those rules & there are many ways to skin that cat, some better than others. When you're busy you don't have time for perfection (as in finessing a situation) - safe & reasonably close is good enough.