My thoughts too Brian, as any engine running time would negate any drain and top up the battery.
The numbers are copied from a generic boat, possibly sailboat.
Plus having 2 batteries where you can emergency parallel should you flat your starter battery is a good get out of jail card.
I have just added a second battery, but for specific reasons have not split them as starter and auxiliary (1, 2 or 1+2). Which is fine having had a single one for 10 yrs and I have the ability to manually start the Yamaha.