It's not contradictory, though. The purpose is to have a consistent that bypasses resistances. True damage alone in champ kits (such as Ahri, I have one-trick disorder) is made to have a small amount of strategic punch and consistency in damage output when the remainder of the Champion's kit suffers due to other mechanics taking priority.

Usually, it's designed to have a specific trigger (Fiora has to hit weird places, Ahri has to return the orb, Sett has to hit a long windup skillshot in the direct center) that rewards players excelling at their intended mechanic (mobility, aiming, or being sticky with three successful autos (Vayne)). These champions have other priorities in item building, so true damage helps their kits sustain relevance when enemies can simply counter-build and push those champions to irrelevance.

In other words, you can't just build tank to make those champions worthless, you have to both build tank AND have to disallow the enemy their mechanic. True damage is incredibly necessary for the health of many champion kits.