It's probably more than just a rumour, at least that's the impression I got from the Finnish media, which tends to be fairly well informed. Meaning that the OS exist and there are products planned, but of course no guarantees that a product will ship.
As to how the Meltemi-stuff make any sense:
At the level that Nokia makes decisions, the smartphone segment of mobile business isn't about hardware anymore, it's applications and services, or probably more to the point, it's about attracting developers. Nokia ditched their own OSes because they knew that by themselves they could not attract enough developers to build a fourth "ecosystem" (iOS, Android and WP being the there current ones). Nokia said that they chose between Android and WP, and, while we can speculate why they chose WP, one of the stated reasons was the fully-fledged and mature tool-chain that WP has.
Meltemi itself may be about many things: hedging their bets, getting something out of the Linux experience they have, or maybe they just feel that the segment suits a Linux-based OS. The next generation of sub-$100 phones will be much more powerful then previous ones and it would be misleading to think them as having very low specs, but it will still be a distinct segment, separate from the smartphone segment, especially it will not be driven by third-party applications and services. That means that Nokia can still, by themselves, make a competitive phone to that segemnt without having to build an ecosystem.
In summary, Nokia ditched Linux (MeeGo) on smartphones because they had to, and they are using Linux (Meltemi) on feature phones because they can.