I don't think it was an inside job. From what I understand, he basically flooded the server and got the site to accept a decrypted server seed which was already known instead of a new encrypted server seed and this way he knew the results of all the bets beforehand.
I am not telling that it was an inside job, but at that time it looked like an inside job by the way in which he clearly knew how the system worked but if i remember correctly i think the person behind Hufflepuff tested the platform during their trial mode run and that is how he figured out how the system worked and he took a chance and he found that the team did not change those settings when the site went online. Only Stunna and the team can verify that as it was a long time back and i do think that is what Stunna said during that time.