You might have a problem if you take the screen shot on a Retina display and then try to use it on a "normal" one, but I don't think so -- we'd hear a lot more about it if so! IMO KM is smart enough to either use the image's metadata and display characteristics to compensate or, more simply, it processes everything to the nominal 72dpi "standard".
This is interesting, and it is definitely not like that on my system (Intel iMac 27" Retina, Sonoma 14.4.1, KM 11.0.2):
Example:
When I take a screenshot of a region, open it in Preview at "Actual Size", it is displayed twice as big as the region on my screen. The info says 72dpi.
Using this screenshot in KM Find Image fails.
Now I set it to 144dpi (without physically changing the size), and Find Image works.
It may be that they add Exif metadata differently, as that's where suggested DPI is stored
To my knowledge, the DPI metadata can be stored in the PNG's EXIF chunk or the pHYs chunk (or both). Actually, the unmodified screenshot (⇧⌘4) does not contain any resolution metadata (in neither of the two).
(Setting the image to 144ppi in Preview writes the information to the pHYs chunk (eXIf contains no related information).)
Now, I think I remember that this wasn't always the case, i.e. that some years ago screenshots were automatically saved at 144ppi (if Retina display). I don't know when or why this changed (or if it is just my memory failing and it was always like that).