It seems likely we’ll never really know. I am only aware of two samples surviving: the first, a remnant that was brought to the London Bar Show back in 2009:
Until about a year ago there wasn’t a person alive who knew what Boker’s bitters truly tasted like. But then a man turned up at the London Bar Show with a finger of the old stuff to share. Made using cassia, cardamom, and bitter orange peel, Boker’s was once swirled with brandy, orgeat syrup, and lemon peel in a cocktail known as the Japanese. The company disappeared a century ago in the wake of legal changes that outlawed many bitters, and it’s impossible to find anything but empty bottles today. “It was a tiny amount of original Boker’s—no one knew it existed,” says Charlotte Voisey, a champion British bartender who has created cocktail lists for The Dorchester in London and New York’s Gramercy Park Hotel. (https://www.departures.com/lifestyle/wine-spirits/bitters-truth)
I’m not clear exactly where the dubious assertion that Boker’s was “made using cassia, cardamom and bitter orange peel” originates from, but it has been hanging around causing trouble for years. My guess is it comes from old book formulas for compounding ersatz stuff.
Later, The Bitter Truth guys managed to score an entire bottle of the stuff, and their “Bogart’s” product is their attempt to reproduce it. They roped in Ted Haigh and Jamie Boudreau (who has made his own earlier attempts at Boker’s speculation) to lend credibility to the project. At any rate, what The Bitter Truth is marketing diverges significantly from the rudimentary description, above. Otherwise, your conclusions are as good as mine. There’s a thread for discussing that product.