What part of the patent in your opinion infringed by the youtube video?
Robot arms have existed long before 2015. And a lot of them use some combination of encoders. The term "secondary feedback" by itself without clarification doesn't really mean anything specific, and in terms used by the patent I would call this more of adding primary/primary feedback system.
The part that the patent seems to repeat is having secondary position sensor attached to the mechanical joint of robot (I assume as opposed to encoders already builtin into the servo drive), although patentability of even that seems somewhat questionable in 2015. I am not that good at reading patents, so maybe I am missing the actually relevant/novel part of that patent.
In the video both encoders are builtin the servo, instead of attached to the arm itself, even more the extra angle sensor introduced by author is attached directly to motor before the gearbox and it's slop which is complete opposite of what the patent tries to claim. The angle servo attached to output shaft after gearbox is what all hobby servos have.
If you go through the actual claims of patent most of them are not applicable to the video. 1) "system for large-scale assembly operations, ... secondary feedback mounted to joint ...". Not suitable for large scale asembly operations, no feedback attach to joint, both feedback systems are built in the hacked servos and can't measure any slop within the joint itself or servo to joint connection. 2) angular accuracy of 0.05 arcminutes - very unlikely 4) system of claim 1 wherein the manufacturing assembly is an aerospace assembly - no aerospace assembly making here, 5) 6rotary axis and 1 linear axis - no linear axis, 6) secondary feedback system is optical encoder -> questionable whether the optical angle sensor attached before gearbox matches the definition of "secondary encoder" as described by rest of the patent, also optical encoders is typically used for describing relative postion/angle sensor based on bunch of slits and counting pulses instead of analog amplitude measurement which gives absolute position. Typically I wouldn't bother with minute differences in classification of how the angle sensor is implemented, but since patent explicitly lists very specific sensor technologies I guess it matters. Otherwise they could just claim that there is an angle sensor/encoder. 7) secondary feedback system is inductive encoder - no inductive encoders here, 8) magnetic encoder - no magnetic encoders, 9) secondary feedback system is resolver - no resolver here (as in analog angle sensor based on ac coupling change depending on angle between two parts to directly generate the sin/cos of angle), 10) "system for acurate large scale-manufacturing assembly operations, ... >3 axis robot arm, with end tool, secondary feedback mounted on rotary joint" - this just more or less restates claim 1 only this time mentioning >=3 instead of >= 6 axis for some reason and mentions an end tool. Is ballpoint pen an end tool for large-scale manufacturing operations? Also the secondary feedback thing discussed before.