I dont' see anywhere it says that mounted MUST dismount to enbark.
On page 10 it says that "Others can dismount only if....(b) to embark,....."
Paeg 10 lists a number of times mounted can dismount - some can do so as listed in the army lists. If not listed they can only do so as per the 4 cases (a), (b), (c), (d).
Secondly - some naval elements can carry mounted troops - clearly then there is no need to dismount these.
However most naval elements cannot. However the first rule you mention allows you to dismount any mounted troops to their foot type in order to embark the dismounted element (within the limitations of hte dismounting rules eg yuo cannot dismount elephants)
Eg is you had an army of Kn that cannot normally dismount, and some ships that are allowed to carry Bd, then you can dismount the Kn as Bd(S) and carry them on the ship(s).