I think the video is not as timeless as his first one about the daiquiri - but anyway - it rocks.
But there is also some more to say about your humble julep...
The first you can find is, that julep comes from the Arabic jallab - as they had a drink, made out of grape molasses and dates flavored with flower water [and all]. I think this reference is rather unimportant - you won't always refer to Karl Benz and his first car, if you are talking about a modern automobile, will you?
But it is rather essential to understand, that this drink is already quite some time around. It predates the cocktail  easily for at least half a century - maybe even longer.
The notion to say, that it is a predecessor of an Old Fashioned cocktail I really cannot share. You have to understand: most cocktails were very similar - smashes, juleps, slings, sours - often only one ingredient [sometimes only the proportions and/or the method of preparation] set them apart of each other. But definitely Jeffrey is right, when he says that it is not at all a bourbon mojito - it simply isn't.
One very interesting tid-bit: while most historic whisky drinks are originally made with Rye whiskey, the Mint Julep is a Bourbon drink. This gives you an indication of its origin - yeah the good ol' Mid West.
The taste? Like a slightly minty sweetened Bourbon. Delicious, simple, boozy, good.
Enjoy Jeffrey's video: