"I can't be certain about anyone else" does not imply "all non-self consciousness claims are equally uncertain". absence of certainty and the absence of evidence and all that.
your "possibility" word is doing a lot of work there I think. you should add "rocks" to your list as well and you'd be equally correct, but we're evaluating the candidates here
Proposed categorization: "definitely not conscious", "maybe conscious" and "definitely conscious". All living things belong in "maybe conscious". Each person is sure that they belong to the "definitely conscious" set, but people cannot prove this to each other. Their empathy causes them to add other people to the "definitely conscious" set. Many choose to add animals to that set too. Some add even inanimate objects to it.