This feels like pure numerology to me, with the entire argument being based on some vague numeric coincidence. I have no experience in food manufacturing, but I'm willing to bet there are much easier ways to mix dough at any desired proportion than the one described here.

Here's another, potentially simpler explanation: I am no pastry chef, but it seems these recipes often use a 2:1 mix of different types of chocolate (e.g. dark and milk) [1]. Now if the recipe contains 1/4 of one type and half of this of another type, that's 25% + 12.5% = 37.5%, which is about as close to 37% than 1/e. Ta-da!

And this is just one explanation I just made up on the spot; I'm sure one can find many others by playing around with the numbers. I don't know for sure what the real answer is or if there even is one (it could just be that marketing thought 37% sounded more elaborate than a round number like 40% while also costing less to produce and tasting the same), but one shouldn't forget to apply Occam's razor here.

[1] a quick Google search gives me this: http://www.talkfood.com/forum/showthread.php/3230-37-Chocola...