@m well it's not that posts are called notes but that of the types defined by ActivityStreams some interfaces have chosen to mark content that way.

As per the standard a note "Represents a short written work typically less than a single paragraph in length."

Arguably this that I'm typing here is a document, not a note, regardless of what Mastodon thinks of it.

https://www.w3.org/TR/activitystreams-vocabulary/#object-types