"Incorrect. You just have to buy the right Inon, made for the corresponding Sony."
Can you please let me know which model that is? I'm kind of curious now, didn't know they have a newer wide lens. I just looked at their lineup and don't see anything that hasn't been around for 5+ years, so it's all geared towards compacts with minimum effective focal length of 28+ mm.
INON Attachment Lens [Lineup]
(btw I know their compatibility tables say the lenses work with M3-M5 models, but any -28 series wide angle wouldn't work with M5 at wide angle end of the zoom without heavy vignette) I even get minor vignetting at 28mm with my UWL-H100 on M2
They would vignette on RX100 4/5 and require zooming in to 28mm which negates much of the wide angle view.
Btw, my general view of cameras is you buy the cheapest tool to do the job properly. I get the feeling with many Sony cameras of late, they cram them with features which are akin to putting $500 racing tires on a minivan. $1000 for a compact is overkill because you can just step up to M4/3 for that price, or heck...I even got a Canon 6D body for that price and enjoy much nicer above water IQ