Oda is a constant tease so he wants us to think Yamato isn't joining just to hit the hammer next chapter
Since Yamato wants to live like Oden she wants to leave like Oden as well
Which means letting the Straw Hats leave in order to hook the chain on
And just like Izou hung on Oden during that scene
Ol' Carrot will be doing the same with Yamato