It's not a matter of assuming, it's a pretty serious point: Is there any test you can imagine that will prove anything about another person's consciousness?
If not, then any discussion about consciousness is "handwavy" and "untestable."
Tests about your own consciousness are interesting, but ultimately can't answer anything about consciousness in general, because they are by definition n=1, since you can't know if my results are actually the result of consciousness.
But I don't think this makes discussions about consciousness useless. If you tell me something about your consciousness I can check if it fits my experience, and you can test something I say for yourself.
What this means is that there might not be an absolute truth about this, only relative truths.
But then, this applies to anything, as no experience can happen outside of consciousness, so all of science is the same way.