The idea that because he wasn't drafted, or didn't make a team, means he didn't have NFL talent ignores a ton of factors, like the right player needing to get on the right team which has the opening at the right point in time... or injuries that are over-estimated... or college-scheme-effects that are over-estimated... or – most importantly – the plain fact of human error in trying to predict the future based on the past.
Your argument implies that teams don't make mistakes, which is obviously untrue. The NFL is perhaps a better talent-sorting mechanism than most, but it screws up all the dang time. Tom Brady drafted 199 in 2000; Joe Hamilton drafted 234 in 2000; to name an obvious comparison pair.
This is akin to "you are what your record says you are." Sure... yet the 10-0 loses to the 0-10 team now and then.