The implication of "true contralto" seems to be that the singer's voice is naturally pitched in the contralto range( say from F below middle C up) and has a warm round tone to match. Those who dont earn the "true" adjective can sing down there but have extended their usable range down by training to a pitch where they are not operating so naturally, and never quite attain to the tone the critics are looking for.You'ld probaly find opera people who'ld get seriously into this sort of thing, but it's not a subject to excite folkies too much I would think.