|Maverick deals with cars, so they may want to verify that the new vehicle owner has a valid D/L before they approve whatever it is that's being approved.|
As far as I know, a foreign passport is valid as ID in most states for notarization, and as others have said you have to go by YOUR state's requirements. But you're not doing a plain vanilla notarization - Maverick is within their rights to demand a specific type of ID for their type of transactions. That doesn't invalidate your notarization, it just means they won't accept that type of ID to close their transaction.
Has your husband asked his contact at Maverick WHY the passport is not acceptable?