Hi Neil
I think this operates in the same way that Naval always worked in DBM. The troop type in brackets is the type replaced by the Naval (as well as being the troop type carried). e.g. taking the Bt(X) replaces one of the Art(S) available on the list. In most other lists you also have to buy the troop type being replaced as crews can disembark, however in this case I don't think you do per rules.
So if the list only allows 1 Art(S) than you can either buy this normally for the army to use on land, or buy the Bt(X) - not have both.
Regards
Richard