mmmmm sorry but Big John's tweet seems confusing.
If a fighter CANNOT defend by himself is because he is not in full control on his consciousness to defend himself (unless some extraordinary event happens, like Silva braking his leg).
So, the KO happens when a fighter is incapacitated to defend himself. A TKO happens when the fighter is still in control of his consciousness but unable to defend. So, consciousness is indeed the dividing line.
In boxing is clear, as a KO happens when a fighter is unable to continue fighting after the count of 10 seconds (you can see fighters who are standing at the count of 10 but unable to defend themselves, and is still a KO).
But in mma there is a thin line due to the non existence of the count of 10, so the only person who can say if a fighter is overwhelmed and is not defending or if he CAN´T defend, IS the referee.
I don´t know if Herb said something after the fight McGregor-Poirier. Watching the video, he jumped to block further punches to McGregor, who was totally unable to defend and with the face exposed to more punches, but when Herb puts his body between them two, Poirier stopped punching, most probably noticing that McGregor was out, even for a few seconds, and unable to defend himself.
At the UFC's website (which is probably as official as we can get), the result of the fight is as follows:
EDIT: To add that to me was a KO.