Hello Orvis, welcome to the forum. There are different coal burning units with diriections and needs for the flue..
All chimney flue pipes should have a slight uphill slant towards the chimney.. The units you read about that have statements about the fly ash are the AHS boilers, and they are a 'different animal'. This boiler has a fan-forced exhaust, and has a cyclone-ash separator in the boiler.. if you have too tall of a vertical stretch out of the boiler, the fly ash that made it through the separator would drop back into the boiler's 'ash cone' and potentially plug it.. AHS wants a horizontal [a slight uphill slope will make zero difference] above the breech to collect any flyash suspended in the exhaust when the fan stops.