One possibility is that perhaps it was actually distributed as part of a bonus disk. Discmaster also seems to collaborate this; there is a single match for that version available here which seems like it originated from a bonus disk but the document has been modified and the self-extracting ZIP file has been rezipped into a regular zip file. Both modifications made it difficult to be 100% sure that was the case, perhaps someone had just dropped the bonus disk readme from some other floppy into that zip file?
Well, I recently came across this disk.

Except for knowing that it was very old, I had no idea what would be on this very disk. I was expecting it to perhaps include the shareware version of Kroz, Pharaoh's Tomb or some other very early Apogee game. And I was prepared that it be difficult to read since the label looked kind of rough and in my experience such disks have also often suffered some sort of data corruption.
However, it turns out it was an Apogee bonus disk from early December 1990! And it was perfectly readable. And includes this early December version of Keen 1!

The disk label suggests that the disk was mastered on December 7th, 1990. All file modify dates are set to November 10th, 1990 which obviously makes no sense but messing with file modify dates was common Apogee practice (sigh), sometimes setting them to some nice round date in the future and sometimes to the past... December 7th, 1990 is thus probably when this disk was mailed to customers.
So it seems that the Keen 1 early December version was a perfectly legitimate release that was at least includes as part of a bonus disk sent to Apogee customers. Thus it is probably safe to say that this is indeed Keen 1 Version 1.0 and not a leaked beta version.
Download Floppy Image (IMA and Greaseweazle RAW dump):
https://mega.nz/file/5w0XlbJZ#y0wLWPAmo ... yvFe0vwElg
Download Scans:
https://mega.nz/file/418SgCiC#ywVXTibNk ... yxm9WY1F0U