If I am not mistaken, it is about the same probability of hitting a 1000x Plinko ball. Not that rare if you ask me to categorize it as, 'statistically unlikely'. It all depends upon the number of total bets placed by any player.
One more thing in my opinion, winning 15 hands in a row in Blackjack has even less probability compared to losing 15 hands straight because of house edge (feel free to correct me if I am wrong).
You asked AI what is the probability of 15 straight losing hands but the player in question has won 15 straight hands , I can understand as we usually have the habit of calculating from the loser's perspective