record AppleTV output with USB Type-C cable and Quicktime. Are there no restrictions? DRM? I followed the instructions here and was able to record the AppleTV output. Awesome feature. I tested Youtube, Periscope, a few games and some German Government TV channels.



I don't own pay apps (like Netflix) and couldn't test. Does this work always? Is there no DRM or other limitations? Can the developer of the app disallow recording? How does this work?
Netflix implements High-Bandwidth Digital Copy Protection (HDCP). The article you cite specifically mentions that HDCP content won't be recorded.  

For an explanation of HDCP - including various potential circumventions - see Wikipedia.