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Of course it's possible, but both those examples are specific applications where dimensions are more important than acoustics. I'd guess in most rooms the depth of a stand mount speaker is rarely a problem.
The problem is semantic really, these speakers aren't designed to be on a bookshelf, just everyone calls them that.
dbr-
^what he said.
I know everyone thinks that their solution is worth sharing but; I have some old Celestion UL6s on 30cm deep shelves that I use as nearfield monitors. They're only 20cm deep so can sit angled in on the shelves. No rear port (they have a passive "bass radiator") which means they're okay with the back facing a corner. Very open and three dimensional. Best speakers I've ever had anyway. Love them. https://www.lencoheaven.net/forum/index.php?topic=13368.0
Dooks-Fatberg
Sure...
But here are in-wall speakers, the KEF Architectural ones and it's an 8" (20cm) driver https://uk.kef.com/products/ci200rs which only requires 4.7"/12cm of depth in a wall.
It is clearly possible to produce high quality speakers with shallow requirements on the cabinet, it only appears to be a choice to not do so.
Even the PSI Audio's on my desk have 4" (10cm) drivers and only have a depth of 6.7" (17cm) on the cabinet.
It's hard to even find great passive speakers which are shallow enough for a bookshelf and yet could fill a room with expansive and high quality music.