Well some in that list have been taught, some others at least have been nurtured in a music heavy environment. Gurutvam is a broad concept. Grammar is NOT the end goal. But even trail blazers have a significant adherence to it!
Also DKP could be a good example of a full tone, where that dominates in the absence of pleasing timbre. And much of the tone shines also because of her extreme syllabic awareness (laya yOga) - which lends a perfection to her renditions. kalpita sangIta has a manOdharma to it - and without much ado. So only insiders listened to her then? She had stature and had good listener support at least in traditional circles! Many others did not.
Such styles cannot develop now , as listeners have gone helter skelter! The cream of listeners show up in the season from all over the world and many of them want to perform themselves. Who will then listen and accredit? That work is a year long full time work.
And the two males that you mentioned had the entire delta region to play with as they could travel easily across villages , unlike Women artistes of those days. And the whole region responded to them. There are people who have listened to GNB's tOdi in the train on request, still sharing stories about that in Dallas. And these are NOT musicians. They attributed it to : " what else one could do? There was no other entertainment then!" We now see it NOT as a mere entertainment.