My guess is that the dampening coil is filled with water. From Building a Portafilter Pressure Gauge
Allow me to clarify... they're called vibratory
pumps for a reason:Image courtesy of Made in the GARAGE
How much of the 60Hz back-n-forth is reflected in the gauge depends on the machine. For example, if it has a gicleur (orifice) in the grouphead, the gauge will vibrate significantly less. The sensitivity of the gauge itself also comes into play. To dampen needle flutter, manufacturers use a coil of thin copper tubing or snubber (e.g., an inline porous stone or dampening piston).
As an example, the needle on the portafilter pressure gauge I made from Ace Hardware parts vibrated like a hummingbird's wings when used on my Rancilio Silvia. It buzzed on La Valentina (E61 with gicleur), but was readable. Of course the Scace II has a quality liquid-filled gauge and its reading is rock solid.
Rotary pumps also generate pulses, but they're barely noticeable for most air gauges with no dampening.
That is, to avoid needle flutter, espresso machines need some dampening agent between the pump and the gauge if the gauge is air filled (instead of oil /glycerin filled). The gauge in the video is clearly an air gauge.