No you are correct as to your recollection. Thanks for reminding me of Eric's site reference.
This is exactly why I like people to post links or basis to their assertions. Most often it is the stuff of urban legend, and vague recollection gathered from the Internet, construed as facts. For instance, there are a number of folks convinced an HD20 can be used with a 128K based on an incorrect claim on a reputable used Mac site.
In this case, Eric's claim is based on a single odd example taken from eBay in 2004. He sent me the picture of the listing a fe years ago, which shows the back of the M0001E with a 1987 copyright date, the front clealy that of the Mac Plus without the label. What's odd about it, is that the box shows a beige 512Ke with the small keyboard. So if this is an original Platinum 512Ke, and not an upgrade of some kind, then Apple did not update their marketing materials to reflect the updated appearance. And most assuredly it was very short-lived, and therefore quite rare (if not the rarest), which explains the paucity of sightings. So until this is confirmed by evidence of an additional Platinum 512ke, I would take it with a grain of salt, but it is certainly a reasonable base upon which to expect this model may exist.