It sounds like they use an FPGA for video processing which means as improvements are made with regards to codecs/compression, they have the advantage that they can upgrade the "hardware accelerated" via software. That is definitely a strong advantage as "hard coded" decoders are not field upgradeable and will age into obsolescence faster.
I know nothing about Arecont's software, but I find it extremely unlikely they do not use a Linux-based OS. They may very well use a distro with a very small footprint, and customize it to suit their requirements while also locking it down, but IMO it would be irresponsible for a camera manufacturer to waste their time actually developing their own OS. There are no licensing costs to Linux or the vast majority of technologies required to implement an IP camera (excepting of course h.264/5 which Mobotix avoids by using their own proprietary codec)
They would ultimately have to write their own web server, write their own HTTP/S implementation including SSL/TLS, SOAP protocol handling for ONVIF support, and that's just to name a few. If they actually did this, then I would consider the software less secure and reliable as it would have had much fewer eyes on it since it would probably be closed source.