perhaps those external stimuli are always present in some form, even if they are extremely minimal, indirect, or invisible to us at first glance.

I still think the connection between external stimuli and consciousness are a lot more glaring than what you realize. After all it is how we medically determine consciousness in humans. Our ability to process and respond to external stimuli is a direct reflection of our somatic and autonomic physiology.

We could even say that it may be impossible to isolate the mind completely from all stimulation in the first place.

You can, it’s called anesthesia.

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.

Its interdependent on your period of development. In gestation unborn infants produce more grey matter than what is really needed to sustain life and can get by with internal stimuli and the external stimuli provided by the womb. However as we develop we become more dependent on higher and higher levels of interaction with our environment to develop our mental capacity.

If a person were to be put under anesthesia since early development or e acquired a tbi that made them unresponsive at an early age, they would all likely mentally deteriorate to a persistent vegetative state.