Klaus called for a retreat, not because he was being pressed, but so that his warriors could recover and wait for when they would be deployed again.
That is still too early.
The reason for calling for a retreat of over 3 million warriors was to make way for two people: Aoi and Lulu.
It had already been 30 minutes since the start of the battle. The first five minutes were spent farming resentments for Aoi's Endless Sword Domain of Slaughter while Lulu farmed cores and life forces for additional swords, as well as making 50,000 more.
They used these gains to strengthen themselves, and when the mercenaries started appearing, Klaus decided to hold back part of his larger force. Lulu and Aoi would fill in for that.
Naturally, when they stepped in and unleashed their attacks, the entire sky turned dark. Lulu outright activated all 250,000 swords, and Aoi also opened the Domain of Slaughter, causing their surroundings to be filled with swords.
