Refresh rate ≠ frame rateNintendo has shown it can provide similar experiences yet go from 60hz to 30hz from Docked to Handheld mode, respectively as Bowser's Fury has done.
The refresh rate is the number of times the display is refreshed per second, whereas the frame rate is the number of frames a system is able to produce per second. The refresh rate is dictated by the display, whereas the frame rate is dictated by the CPU and/or GPU.
Both the Nintendo Switch's (and the OLED model's) display, and the TVs currently on the market, support a refresh rate of 60 Hz. Digital Foundry is talking purely about frame rates when talking about the differences between handheld mode and TV mode for Bowser's Fury.