The Philosophical Zombie Argument asserts that there is a possibility that everyone in the world other than one person doesn’t have consciousness; even though their outward appearance and behavior is identical to a conscious human.
One what grounds is it possible? A lot of these thought experiments like the brain in the vat, the evil genius, or “it’s all just a dream” all rely on the premise that if we can imagine something is possible, then it really is possible.
But that’s a ridiculous idea. Imagining $1 million in front of me does not make it real, and so imagining the possibility of these things does not make the possibility real either. It’s magical thinking, plain and simple, even if we dress it up in philosophy.
If someone wants to establish that something is really possible, they need to provide evidence for it. It is not rational to deny the evident by the non-evident.
That means, even if these “zombies” frequently have intellectual conversations; these are just random body reactions that just happen to appear like conscious conversation. It is just a freakish coincidence.
The way we know that others are conscious is by looking at how our interior life relates to our outward behavior, and by observing and discerning this outward behavior in others, we can determine that they too are conscious “on the inside” as we are, and even go so far as to figure out what they are experiencing “on the inside” based on drawing an analogy to our own interior experiences (for example, we see how crying is caused by sadness in us, and so we can discern that someone might be experiencing sadness if they are crying).