In my opinion, that's probably a good thing; less baggage that has to be maintained for POSIX compatibility.
Remember that Unix was originally designed for large, centralized timesharing systems. It has been adapted to the embedded realm throughout the last couple of decades, but starting from scratch to create an OS specifically for the embedded realm is ideal.
> may be probably cheeper to use something like a full-blown small computer (like the rasperry-pi) with Linux on it.
In some cases. In the case mentioned by the article, however, a Raspberry Pi (for example) would make for a rather large badger collar. IoT is indeed a pretty broad and vague concept right now, but it already does demand very tiny hardware with size constraints dictating available computational capacity. That's where Contiki comes in.