I was just saying that if one can remember sed syntax, Vim's should be easy to follow.

You don't need to memorize the syntax to have a script.

No joke: I wouldn’t have a programming career if remembering syntax were a requirement.

The author might just have copied that sed expression form someone else.

Or looked it up once and probably now forgotten it.

Also regex expressions are not vi expresions and have wider applications so author might use them regularly.

So nothing here suggest that the author can remember vi.