He was buying himself time. He couldn't win a direct confrontation at the time before (assuming it's ok to give spoilers now) he had zero of the other regents on his side. He had his brother against him, Ishido, and the two Christian Daimyos. If he acts like he is going to fight, then he tempts them to mobilize their troops across the country and force him to fight his entire way to Osaka.
By looking like he is surrendering, he is allowed to bring his entire army across the country unopposed, while giving himself months to manipulate the other regents to his side.
The last reason is that the entire society is set around manipulating the intent of politeness, honor and codes while strictly obeying the letter of these things. Similar to how Ishido had all the women as political hostages and was murdering anyone who tried to leave, but had it framed as protecting them from bandits.