I would say that questions about the BIOS are off topic, yes. As you correctly point out, the BIOS isn't part of the OS. However, the question you linked to isn't really about BIOS but about how one can upgrade the BIOS using Ubuntu. It is, therefore, a question about Ubuntu falling under the general category of "How do I do foo on Ubuntu". So I would say it is perfectly on topic.
A rule of thumb could be that if the question is asking about configuring the BIOS, so that the answers wouldn't involve Ubuntu at all, then it is off topic. However, if it is asking about using Ubuntu to do something, whatever that something is, then it is on topic.
Finally, I want to address your point that "Ubuntu related answers could as well apply to any other Linux distro". That is not relevant in any way since every single answer on this site would also be applicable to other distributions. There's nothing Unique about Ubuntu: Ubuntu is based on Debian and many other distributions are based on Ubuntu. This means that anything applicable to Ubuntu is bound to also be applicable to at least some other distributions. There are very few things that are specific to any distribution, let alone one as widely used and built upon as Ubuntu, so whether a solution is applicable to other distros really isn't a consideration when considering what is on topic. Also see: Are "not only Ubuntu-specific" questions on-topic?.