That is absolutely not true. You can write a Raspbian image directly to the USB drive and copy the files from the "boot" partition to a FAT32 formatted micro SD card, and with a simple file edit it will boot fine (all you need to do is remove the command that launches the resize script from cmdline.txt). I've been meaning to do a full tutorial on this, but I've been dealing with some "life" nonsense and haven't had time.
Simple version of hybrid SD-USB boot tutorial:
- Write Raspbian image to USB drive with Etcher.
- Edit cmdline.txt to remove the resize script launch (see edit below).
- Copy files from USB "boot" partition to FAT32 micro SD card.
- Plug card and USB drive into Pi4 and boot it up.
Then, of course, you'll need to resize the 2nd partition...
Forgot to mention, when you remove the resize command from cmdline.txt, make sure you only leave a single space.