TLDR?
Bob is saying that the actual defence/attack stats don’t ever drop in effectiveness, having 20k defence is 20k times more effective (in theory, not against bosses) than having 1 defence.As for whether or not attack/defence plateaus, in short yes, in long answer, no.
--Bob
However, because mobs and bosses have limited attack and defence, there is a point where you will have more attack or defence than anything else in the game, so more would no longer benefit you. That is roughly around 15k, but 10k is usually good enough.
Hope this helps