That is nonsene, at least in such a generalized statement! Relics have shifted things a bit but rather in terms of making flat offense stats less useful than %age increase.
Let's assume that you do get +20% offense from primaries and secondaries, which either means very good secondaries without using an offense cross or rather bad secondaries with an offense cross (assuming 6E mods). In that case your base damage is either 120% (CD set) or 135% (offense set), or in other words, an offense sets does 12.5% more damage on a normal hit. If both sets have a 6E CD triangle, the situation on a critical hit is as follows: 120% x 2.22 = 266.4% (CD set) vs. 135% x 1.92 = 259.2% meaning that a CD set has a small advantage of about 3% more dmg. In this case an offense set might in fact be the overall better choice.
However, let's look at the situation in a SE squad which gets +50% CD, which generally favors offense mods but also has ferocity, which (like all other offense bonuses) tends to favor CD mods. With 10 stacks of ferocity being active on average, the CD set on a critical hit is at 200% x 2.72 = 584.5% as compared to 215% x 2.42 = 520.3% with an offense. Here, the CD set does 12% more dmg on a crit wheras the offense set does 7.5% more dmg on a normal hit. Given the high crit chance in that squad a CD set is definitely the better choice.
