Uncomfortable truth bombs:
Shanks didn't come to save the alliance.
Shanks & his crew came by to collect Marco and drop him off at Sphinx island which they did the very next chapter.
Marco even says that his "ride" came to collect him.
He sees Broccoli man trying to pull some shit on an exhausted alliance (his words, not mine) and humbles him right there.
If Broccoli man had fought the alliance, he'd suffer far worse.