I imagine they were refering moreso to diminishing Value.
And by that I mean personal value, not a number value.
40% crit is well and good, but is pumping stats towards 41% once you already have almost 1/2 chance to crit worth it when you could be adding into straight spell damage, which makes said crits do even more?
Much like eating food, or live in general, everything in moderation. Stacking one thing and one thing only leads you no where