What is the intended use of the 400 Hz-rated power supply on osciloscopes?
Yes that’s what it means. Planes often use 400 Hz generators and the plane’s frame becomes “Earth ground”.

As always, Gnd just means 0V anywhere in the universe and Earth bonded implies 0V to earth ground with some impedance &lt;100 Ohms each to another grid location with an earth ground, unless in dry remote areas with poor earth bonding.

Earth is used  by all 3 pin instruments to bypass EMI line filter leakage currents only and provide a good low impedance reference for scope probe ground leads.Sometimes DSO’s will put a plastic cap or an inductor between 0Vdc and Earth gnd pin for various reasons to isolate earth ground common mode RF  noise (L), or block AC current but bypass RF to a relatively low impedance AC gnd using a cap only.

Engineers do use DSO’s on planes for R&amp;D work.