The exact wording we used was, "...the man cannot easily lead the lady to do such an action", which I admit leaves some room for interpretation.
I don't mean to say that a Promenade Develope' absolutely cannot be lead on some level, but it would be very difficult to do without restructuring the technique in some way. Our aim with our syllabus is to reflect as much as possible what people actually do, and so our interpretation and subsequent description is a reflection of the prevailing theory amongst those who dance this figure -- mainly social and casual hobby dancers -- for whom it is generally considered an interpretation by the lady.
According to this theory, the lady can choose to lift the leg (or not) much in the same way she has control of the artistic interpretation of many aspects of her dancing -- arm styling, for example. It doesn't change her shape, or her weighted foot, or anything else that might be considered a "hijack" (which, incidentally, is another technique considered acceptable in certain social dance circles). However, she does need to be aware of how it affects her balance, which is another good reason not to attempt to lift the leg too high when social dancing.
So if you accept the idea that the follower has room for interpretation when it comes to her legs and feet so long as it doesn't interrupt the leader's intentions, then it doesn't actually matter whether or not it is leadable... The fact is, it's doable.
That being said, it would still be interesting to explore the topic of leadability with respect to the develope'. I will tread lightly here, because I've witnessed many heated debates over the years regarding what makes something "truly leadable", and it can wind up being pretty philosophical. But here is a point or two to consider:
People have different standards of leadability. The strictest interpretation is that only the actions that can be directly caused by the leader without any cognitive interpretation by the lady, such as weight changes, can qualify. The most liberal interpretation would be one that includes anything you can make your partner do by any means, such as standing 3 feet away and saying "turn to your right!" Most people's opinions fall somewhere in between these two extremes.
Since a develope' is not a directly-caused action, it requires at least some degree of cognitive interpretation, based on principles agreed upon ahead of time, typically some combination of shaping and elevation. This puts it on the "liberal" end of things, since there's nothing inherent in the man's movement that suggests she do anything other than shape and elevate equally.
If your interpretation of acceptable lead-and-follow includes that, then you would be one who would describe a develope' as leadable. But even if that's so, the syllabus version of the Promenade Develope' will still be more difficult to lead than other develops, specifically because when taken exactly as written, there is little you can do with your shape or elevation that would indicate to the lady that she should lift her leg. An advanced dancer would be more likely to modify the interpretation to allow for more shape and therefore leadability, as in the version I described in a previous post.
Regards,
Jonathan