There certainly seems to be limited evidence that authoritative sources identify an entity known as the Kingdom of Cornwall or at least that they choose to describe it as such per the evidence above and a Google Books search Books I have that might be expected to touch on it or refer to it admittedly limited in number and of a generalist nature do not talk about a Kingdom at this time although obviously it was a separate entity of some sort to EnglandWessex Im no expert myself on this era of history and have no access to specialised sources but as noted the substantive content might seem better placed elsewhere in other pages  and indeed already is