This chapter confirms that you need orders to engage with a Yonkou. Even when that Yonkou attacks you first because of starting a war
Which we see GB also does when he tries to hide his intentions from Akainu about attacking Luffy hoping that capturing/Killing Luffy would out way him not getting clearance
So when you see the Marines standing around, they were waiting for orders from Sengoku, not because they are scared. They were literally fighting WB before that. Shanks also knew he was in an unfavorable position which is why he said LET ME SAVE FACE
Which we see GB also does when he tries to hide his intentions from Akainu about attacking Luffy hoping that capturing/Killing Luffy would out way him not getting clearance
So when you see the Marines standing around, they were waiting for orders from Sengoku, not because they are scared. They were literally fighting WB before that. Shanks also knew he was in an unfavorable position which is why he said LET ME SAVE FACE