- "there's a ship nearby that would be perfect for me to hitch a ride on"
Meaning he just noticed that ship and it was convenient that it was Shanks whom he knows. He did not call them. Shanks is not Marco's personal taxi that he can call over to pick him up whenever.
- Luffy was surprised at Marco leaving so soon
- Marco only came to say that after Shanks made himself known with the haki blast. Not before. Meaning once again that Marco only noticed it's Shanks' ship because of haki burst and not because he ordered Shanks to come
Maybe because Shanks did that haki burst where everyone noticed?
And was Marco at the flower capital? Wasn't he in ringo at Izo's grave?
So why did Shanks interfere? Why not let Greenbull get neg diffed?
It would be another accomplishment to the new generation that he wanks so hard.