Doubting if Speakers were used mounted on a Bookshelf was not my query.
I would have expected to learn of Speakers mounted on a Bookshelf.
My query was with all the knowledge out there today about Room Treatment and Audio System set up, the idea of suggesting a Speaker is intended for a Bookshelf is needing addressing, as Bookshelf mounting is far from a typical recommendation to interface a Speaker at its optimised within the dedicated listening space.
The concept behind dedicated Speaker as a Bookshelf design, suggests a Bookshelf is the preferable option to purchase, in place of a Stand to seat the Speakers on, which with a Stand enables options on multiple positions for a placement within the Listening space.