DCing would count the game as lose and would count the lose towards Bronze 1 -> Silver IV Promo.
What he says he experiences is suddenly becoming eligible for Bronze 2 -> Bronze 1 promo.
If he lost the aformentoined Promo, he would still land at around 75+ LP Bronze 1, not suddenly appear at Promo of Bronze 2 -> Bronze 1.
Might be that he just missed something or a bug.
EDIT: So I misread it - he wasn't in Promo but won towards Promo. It simply placed him into wrong Promo. Very easily could be that he just misread something.