Why does this guide recommend a 500MHz oscilloscope for power supply testing?
The parasitics of 0.01uH (10nH) and 10pF will ring at 500MHz.

The parasitics of 0.001uH (1nH, or 1mm of wire) and 100pF will ring at 500Mhz.

SwitchRegs have several situations where 1mm wire and 100pF co-exist as parasitics. You need to see that, and be aware.

Why? The ability of OpAmps and other ICs to reject 500MHz on their VDD pins is, basically, ZERO.

You, the power-system and power-circuit designer, have to take responsibility for identifying and quantifying and managing the risks.

EDIT Some years back, checking out a 4-channel camera interface, I found the top 1/3 of the 4096 codes went to trash for the 6 LSBs. Bringing in the Tek P6201 FET probe with 900MHz bandwidth (1970 era technology), I eventually saw a 0.01volt PP oscillation at 600MHz on the -5volt power ---- the ADC was oscillating.  Cure? Install 27ohm Rdampen in the (4) ADC input paths; we gently bent up that VIN lead on the IC, and soldered a vertically-oriented SMT resistor. Somehow my 8-layer PCB, with 0.1uF bypass caps diligently placed, with 4 ADCs sharing the front/back, perhaps with ferrite-beads to isolate the VDD, was oscillating. The IC vendor stated "Never seen that happen before."

Without that 900MHz probe and 500MHz Tek7904, I'd have had to punt. Touching the ADC Vin pin with fingers caused changes in the output code-blurring, so I had a hint of weirdities.