In season 17 does LoN buff work against season journey set rewards?

LoN builds are generally not that strong, but it does free you up to use 2 ancient rings to increase the benefit of the passive seasonal buff.

In your specific case, the build guides you are following are just throwing the seasonal buff out the window because the set bonuses are so crazy strong that there's no point in building around ancient items only.

To answer your question in the title of your post, yes, it does work against each other. Seasonal buffs were never thought to coincide with seasonal rewards, these two things were built separate from each other.