dvclmn

Ah!

I’ve seen my error; I’ll write it here in case anyone else was confused:

It’s not url("{@stylesheet_directory_uri}/etc"),

It’s url("@{stylesheet_directory_uri}/etc").

(‘@’ and opening bracket are switched)

This is perhaps more obvious to a seasoned LESS user, but easily missed for a beginner. It could definitely be made clearer in the wiki below though, which doesn’t even mention the curly brackets at all.
https://github.com/oncletom/wp-less/wiki/Common-Usage