But bi means two meaning it literally refers to being attracted to two genders by definition. Sure it's often used to refer to people who are somewhere between binary genders as well by more inclusive people, but it also often is used to exclude trans and non-binary people by more conservative people. What it definitely doesn't include by definition of the word is people who are totally outside of the binary spectrum.
It's like people who call transwomen "dude" or "bro". Sure they might mean that as a gender neutral term in their heads, but it is not a gender neutral term to the majority or by common usage. Or people who use the b word. Sure in some groups of people it's a term of endearment, but in many its meant to degrade women since the literal word means "female dog". So, if some people use the term as an insult then it doesn't matter that you might not be meaning it as an insult, it's still going to feel like one. On the internet you have to be more specific with wording things and not use regional slang outside of communities specifically for those groups.
Similarly, even if you personally use bi to mean all, that's not how some people use it, and it's not the literal meaning of the word, so it is generally assumed by people who are outside the binary spectrum, and sometimes trans people who are often treated as non-human by conservatives, to not include them unless they know the specific person or group means it differently.
For example on dating sites that include both bi and pan, if people choose bi over pan, I generally assume they aren't interested in trans or non-binary people. If there's no other option, then I might assume they mean all.