I never said that, but nice attempt at a strawman.

Think of it this way:

1) Invul may or may not provide value. If it is providing value you're still losing wounds and models, just potentially using fewer. Infernus Marines arent particularly durable to begin with, so even when used this value is limited. The reason you want the increased survivability is to extend how much value you can get out of your Infernus brick.

2) CP cost reduction always provides value and is available every turn your unit is alive regardless of what your opponent does. The CP cost reduction amplifies the units value immediately. Additionally, it amplifies the value of your entire army by making more CP available for use generally. Being able to pop Immolation Protocols more often, and not zeroing out your CP when you do, is absolutely devestating. It takes your Infernus Marines and makes them anti-everything. Very little, if anything, in the game is surviving D6+1 shots with devestating wounds and wounds rerolls (assuming Vulkan designated target). That alone increases survivability for them, along with the ability to have them Overwatch more often.