I'm sorry but no you haven't. Deion Sanders is one of the best CBs to ever play the game. All you said was he was the best cover corner ever and you are saying Gilmore is horrible because he can't tackle. Deion couldn't tackle. Why is he considered one of the best to play the game, if he couldn't tackle?
It doesn't show a complete lack of understanding at all. That is what Deion was paid for. He was paid for covering. Richard Sherman, also not a good tackler, is paid to cover.
If you'd like to talk about how defense work in the NFL today, I'm more than happy to discuss. But why certain payers are paid the way they are is a completely different topic.