From their blog:

Now regarding YellowKey, lots of you are wondering how does one even find such backdoor ?

I’ll tell you how, it took me more time trying to get it to work than the amount of sleep I had in two years combined. No AI involved, no help in any shape or form. I could have made some insane cash selling this but no amount of money will stand between me and my determination against Microsoft.

[…]

I can’t wait when I will be allowed to disclose the full story, I think people will find my crashout very reasonable and it definitely won’t be a good look for Microsoft.

Looking forward to the full story.