You are correct. 5V on the USB port is required for nRF52 devices (Boron, B SoM, Argon, Tracker) because the MCU does not power the processing block that handles USB when there is no USB power as a power-saving feature.
5V on USB is not required for RTL872x (P2 and Photon 2), or STM32 (Gen 2) to use USB.