I suppose they capture the video stream and detects pattern in it. I guess it was just a matter of time as the gaming industry went bonkers with the anti-cheat trojans and eavesdropping, pushing hackers to go for the raw video.

Sadly the side effect will be that the industry is going to push high bandwidth digital content protection (HDCP) on every cables now.

HDCP - https://en.wikipedia.org/wiki/High-bandwidth_Digital_Content...

I never tried as I don't have one, but apparently cheap HDMI splitters can be used to strip out HDCP.