"No buts. I really thought about it a lot. It's a good plan to gather forces and fight a war of attrition, but that isn't the ideal option. That should be a backup plan even if it's one! The primary plan should always be the one which deals with them directly with as little loss as possible."
Instead of listening to Lilith for now, Atlas had made up his mind. He didn't want to have his advantage go to waste. He was the only Progenitor who could use spells to hide his aura. Moreover, it was even easier since his aura wasn't entirely as overwhelming as Castiel's aura which could be sensed even when he used Shroud.
This was an advantage which Castiel wasn't going to get. If he wasted his last week, this advantage was going to be gone forever and he would regret not helping others.