I don't think it is that they are saying that any less than 18,600 is not enough to live on, but are saying that any less than 18,600 a spouse can claim benefits in some form as 18,600 is the threshold.
BUT the whole point is that the Sponsored Spouse CAN NOT claim any benefits as they have no recourse to public funds until they get their ILR. UNless they mean for after they have their ILR??