> When was the last time you heard anybody say "opus" or "sonnet" before this?

Not a lot, but occasionally. I was familiar with the words.

> Also it implies that "haikus" are inherently inferior to longer texts which may be kinda frown-inducing..

It really doesn't.

The one real weakness of Anthropic's naming is that Fable doesn't fit. A fable is typically a fairly short story whereas I think of an opus (if a written work) as being at least a long book and maybe a whole collection of books.