Except it is in the manga. The anime is an
adaptation of the manga. It
adapted this
one panel that shows the contrast between rough waters and the Calm Belt
between these two snapshots (that are literal seconds apart) in the same way it
adapted the conversation between the Nami and the rest of the crew slightly differently.
The anime is secondary canon and can be used when there's no contradiction, which there isn't. What's been shown is not what's considered an "added scene." Something like Ace vs Smoker is, where they extended
a single page Ace blocking Smoker into a
skirmish of several clashes. You and Billy don't know what yall are talking about.