just a guess but s/he may have been referring to just putting root filesystem on a USB drive, but still requiring the bootloader (and possibly kernel?) to be on sd card. It's a one-time "hit" that is completely negligible.

That's the present situation and I've used it before. Some recent Pis can boot directly from a USB drive without the need for a bootloader on the SD card. That's still in the works for the Pi 4 as I understand, but network boot (PXE?) will come before that.