I think this may be out of context...I think this video is for designers not photographers...there's big difference. Illustrator and InDesign really want to be driven as CMYK.
I think Jeff is correct, because the sentence goes: "When
designing for print ...".
Context matters as usual, so when designing in CMYK, it is not a good idea to convert to RGB and later back to CMYK again. Each conversion is prone to add losses, because the color spaces do not fully match. Also, in CMYK colors to be printed can be created in different ways, depending on the settings for the printing press (e.g. dot-gain and UCR settings and ink types and substrate color used).
Digital capture on the other hand are by nature RGB devices and so are the printers used for printing photos on Epson or Canon printers. The printer prefer getting RGB images and does their own special separations.
Yes, and it's a single conversion, from RGB to (usually) many more ink-colors than only CMYK, instead of a round-trip. A conversion from RGB to CMYK to printer ink-colors, is also 2 conversions, and the CMYK step is superfluous and damaging to the final image quality.
So when the image originates from the computer and is designed for a printing press, by all means do start and stay in CMYK (with the correct settings for the output modality). But when an image originates in RGB, like from a digital camera, limiting to one final output profile conversion (either a CMYK separation or a many ink color inkjet output, or an RGB C-print modality) is best.
Cheers,
Bart