Lol, the comparison to Brook literally serves as further reason for why Yamao is likely a red herring.
You can often tell what moments are important simply by the panelling, how they are presented on the page. You could remove the dialogue entirely and still recognise a stand out moment simply by the layout of the page and design of the panel. Just compare the two:
So in one corner we've got the climax of what the whole chapter was building up to. A cliffhanger presented as a double page spread with minimal dialogue and huge onomatopoeia
vs
A slim panel within an overly cluttered page that's seemingly forgotten as quickly as it appears. Even the phrasing is clunky, lol.
You don't even need to know what Nami's saying to be able to tell that it's probably something significant.
And of course as we know, Brook initially declined the offer to join because of personal reasons which were only resolved after an emotional story arc that spanned across all of Thriller Bark. You've got the same deal with every other straw hat.
Yes, it's obvious who is going to join the crew, but that's
not because Oda straight up has the characters state word for word that they are going to join. It's because of the stuff beyond the surface level. Their dreams, backstories, character arcs etc. that make it obvious. In every single case Oda used some form of misdirection in order to conceal the truth.
This video explains it very well and why Yamao's case is totally different from actual straw hats.