It's not unclear at all. Cis means someone who is not trans. That's it.
'Cis' is short for cisgender. The very definition depends on a belief in gender identity for its construction. Therefore, it describes nothing without acceptance of gender ideology.
And the word 'cis' means 'on this side', relative to 'trans' meaning 'across' or, in this context, 'on the other side' (i.e. one's gender is 'on this side' or the 'opposite' side of biological sex).
So yes, if you use 'cis' in the context of 'trans' or 'not trans', I can see why you would see it as simply meaning 'not trans', but if you do this, you would be ignoring the fact that not everyone is either 'cis' or 'trans' - there is a large number of people in the world who reject this understanding of gender identity because it doesn't accord with their lived experience of gender and who therefore aren't caught by either 'trans' or 'cis'.
So it isn't inclusive to use 'cis' to mean 'not trans' as lots of people are excluded and marginalised by this definition.
Many people experience gender in a way that is not adequately described by 'cis' or 'trans' and it harms them to push a narrative onto them that ignores their lived experiences and describes them in relation to what people outside of their group assume they experience, purely on the basis that they think that's what it looks like from the outside.
The impression I get of people who use the term 'cis' is that they've listened to some people who don't identify as trans (for all sorts of reasons) but have mistakenly assumed that those people experience gender in exactly the same way as all other people who appear to be the same as them. I view people who use the term 'cis' as bigoted and hypocritical, because their use of the term requires an assumption of other people's gender identities while at the same time preaching that everyone should be free to dictate their own gender identity according to their personal lived experience and that misgendering other people is offensive and disrespectful.
It's like the when a toddler first learns the word 'cat' and then mistakenly understands everything else to be 'not cat', until they have more life experience and have learned enough about the world to understand that actually, this is a dog, and this is a bird, and this is a capybara. And they keep learning all these new animals throughout their life as their worldview expands. Vets manage to write articles about cats without referring to dogs and birds as 'non-cats' throughout, so I find the claim that intelligent adults can't adequately describe the trans experience without referring to everyone else by defining them as 'non-trans' as utterly bizarre.