Yes, the stated Purpose at the help doc for GetThumbnail does omit the side effect that you describe.
Does it default to 72dpi even if the device is at a different dpi?
For example, the...
...is 264 ppi
Defaulting to the device dpi/ppi or better yet...as you say...having dpi as an optional parameter of the function would be better.
Hmm... that is a good question. I don't know, as I have only tested this on OS X. Will need to test and report back!
In FileMaker Pro, thumbnails are always rendered at 72dpi.
I have alerted the appropriate people to make sure this gets included in future documentation.
With the release of FileMaker Pro 16, the Help documentation has updated the GetThumbnail section:
"The thumbnail image always maintains the proportions of the original image and has a resolution of 72 dpi."