Not false exactly - I think the "Lady with Ermine" interpretation is still the literal reading of the painting. The white coat speaks to that not to mention the current owners have incentives to promote it as such. Reading it as "Obnoxious Merchant's Girl Pretending to be a Lady Seated with a Polecat Pretending to be an Ermine" would be subtext inadmissible without further explanation which I attempted to provide above. Namely: means, motive, and opportunity.
If one does want to be literal about it, the painting was made at the same time as his Vitruvian Man studies. Relative body proportions would have been top of mind for Leonardo at this time. That "ermine" is the same length (if not quite a bit longer) than her arm. Assuming she's conservatively 140cm tall (4'6") - her arm and the "ermine" would be 55cm long (1'9") (or more!). A quick cross reference with typical sizes for these creatures[1] - shows that it's just barely above the size of a typical polecat but more than twice the size of any stoat (an ermine in non winter colors). Proportionally it’s the difference between a chipmunk and a squirrel.
Put another way, if that actually is an ermine, then that makes her 75cm (2'6") tall. Just cross reference the size with the ermine in portrait of Elizabeth I below it. If he explicitly called it a polecat (a creature more associated with its foul smell and ugly temperament than any true nobility - that Sredni Vashtar story is great btw - thanks for that) he would have been out of a job. Or worse. This is the era of Machiavellian politics.
He may have chosen to sleight her in this way because he could plausibly deny it to her face "Well, yes of course that's an ermine, it just happens to be a realllly strong ermine - it's a symbol for your lover, the Knight of Ermine, you see" or if pressed by Ludovico to reveal the 'true' message: "Ok, fine you got me, this creature may have a white coat but it is not true nobility. As you would know my lord, what with her being formerly engaged to your rival and all."
From a Machiavellian perspective, Ludovico may not have cared one way or another. She was the daughter of a very wealthy merchant betrothed to his rival. By taking her as his concubine he deprived his rival of access to capital.
If one does want to be literal about it, the painting was made at the same time as his Vitruvian Man studies. Relative body proportions would have been top of mind for Leonardo at this time. That "ermine" is the same length (if not quite a bit longer) than her arm. Assuming she's conservatively 140cm tall (4'6") - her arm and the "ermine" would be 55cm long (1'9") (or more!). A quick cross reference with typical sizes for these creatures[1] - shows that it's just barely above the size of a typical polecat but more than twice the size of any stoat (an ermine in non winter colors). Proportionally it’s the difference between a chipmunk and a squirrel.
Put another way, if that actually is an ermine, then that makes her 75cm (2'6") tall. Just cross reference the size with the ermine in portrait of Elizabeth I below it. If he explicitly called it a polecat (a creature more associated with its foul smell and ugly temperament than any true nobility - that Sredni Vashtar story is great btw - thanks for that) he would have been out of a job. Or worse. This is the era of Machiavellian politics.
He may have chosen to sleight her in this way because he could plausibly deny it to her face "Well, yes of course that's an ermine, it just happens to be a realllly strong ermine - it's a symbol for your lover, the Knight of Ermine, you see" or if pressed by Ludovico to reveal the 'true' message: "Ok, fine you got me, this creature may have a white coat but it is not true nobility. As you would know my lord, what with her being formerly engaged to your rival and all."
From a Machiavellian perspective, Ludovico may not have cared one way or another. She was the daughter of a very wealthy merchant betrothed to his rival. By taking her as his concubine he deprived his rival of access to capital.
[1] http://deskarati.com/wp-content/uploads/2015/01/012615_1236_...