Was he able to cash out the btc or was he stopped?
He was able to cash out 2000+ btc as hufflepuff but his next withdrawal as robinhood was blocked because Stunna has found out that he is using exploit to win
I don't think Stunna actually found out until Robinhood apparently made it obvious by getting like 140 wins in a row at 50/50 chance , which tipped everyone off. So it might just be him, who purposely did that to show the flaw. But even now, without Stunna's comment on the situation we still don't know if Robinhood was actually Hufflepuff.
Stunna had mentioned above (which have since been edited, but the quotes of his posts remain) that they used the same exploit.
You are probably correct to say that stunna did not know about the exploit until it was made blatantly obvious by robinhood.
It was mentioned above that Hufflepuff had withdrawn to
14iS2UvcLK33xkC1K1qL1dhEbp49aiNfNp, and it is known that robinhood was able to withdraw ~20
BTC before his additional withdrawals were stopped, and there do not appear to be any transactions in the amount of 20
BTC to the above address, so it would be reasonable to conclude that it would be possible they are different people.