Yes, definitely there will be a few that slip the net and but for injuries your brother might have been one. You say though that he rejected a pro contract in favour of going non league but there lies the crux of it, perhaps he just didn't have the desire to chase the dream of playing professionally. That isn't a criticism, that's just stating a fact that having that well worn phrase 'hunger and desire' is a massive part of being a professional, perhaps more than us fans realise. I don't doubt for a minute that there are lots - probably a fair few at the level Oxford City have just come from - that are technically superior to many that have made it a league or two higher up but don't have the fight to comeback from a set back or two, be it an injury or rejection.
My comment earlier on that I didn't think any from City could make it at L1 (where we are) was based more on my observation yesterday that none looked technically up to it at that level - never mind the desire, which obviously I can't comment on. It is quite hard to tell because the pace is so much slower but when you watch a L1 game and then a National League South game it looks a world of difference to me - touch, speed of play etc. Fitness obviously plays a part and that would clearly act as a bit of a leveller if a few at NLS were training full time in L1.