I used to think this too, but turns out, this is a huge misconception.
https://www.youtube.com/watch?v=ZT8vDnWC5UQLooks to me like fanaticism is spreading the curse just fine here. And if there is any marginal increase from holy freeze, I’d argue it’s completely negated by the benefits of fanaticism aura.
Cursed gives monsters a 75% chance to cast amplify damage on every target they damage. That includes the AOE of amp damage, increasing by level, hence a 75% chance to hit a big aoe. And a 25% chance to whiff on each attack, or if that attack is blocked or missed. When you have holy damage auras, that 75% chance to cast amp damage is rolled on every target hit by the holy damage every pulse, which shotguns the aoes to redundantly apply it, hence if aura hits 10 clumped monsters there's a 99.999999% chance you'll apply amp damage to all of them. And against bosses who can have 50% block and your demon might only have ~80-90% chance to hit after high level and defense, the holy pulses can more reliably apply amp faster. Except holy freeze does not hit act bosses, while holy fire/shock do.