Well, that’s interesting. Is that a quote from the AU site? I found this on the US site which implies the air portion can be dropped:
INTERNATIONAL FLIGHTS
The air promotion is only available for the first and second full-fare paying guests. Guests could also choose to upgrade to roundtrip business class paying an additional cost.
In case the booking includes more than two guests, they will need to purchase roundtrip economy class airfare.
Airline baggage fees are not included.
In the event neither Business Class nor Economy Class air is available (determined at Silversea’s sole discretion) or for guests not utilizing the promotional air offer, a non-use Air credit may be applied.
Silversea reserves the right to modify the Silversea Air Programme procedures and fees at any time without notice.