Yes, I have to admit that I don’t have any example of consciousness in an individual completely without external stimuli. In fact, I agree with you on the last point you made. My point is that we need to be more precise about what we mean by “external stimuli” exactly.
We could even say that it may be impossible to isolate the mind completely from all stimulation in the first place. That leads to the real issue: perhaps those external stimuli are always present in some form, even if they are extremely minimal, indirect, or invisible to us at first glance.
So I correct myself, the argument is not necessarily that consciousness can exist with literally no input whatsoever, but rather that what we call “external stimuli” may include things so subtle that they are easy to overlook. In that sense, the claim becomes less about the absence of stimulation and more about how little stimulation is actually enough to sustain consciousness or mental activity.