If you have an old computer which cannot boot from a usb drive, you cannot put the tce directory on the usb drive, as it cannot access the drive early enough in the boot process to load extensions.
Wrong, the boot medium which needs to contain the kernel and probably (not 100% sure if there could be ways around that) the initrd has to be accessible by the BIOS.
Nothing speaks against mounting the tce dir from USB when USB is not a boot option.
Have you done this in real life, or are you speaking theoretically.
This information is to make it simple for new users.
If a person simply puts their tce directory on a usb drive, and the computer cannot boot from the usb drive, it cannot access the drive early enough in the boot process to load extensions.
It is possible to do many things.
Theoretically, it is possible to start Tiny Core without extensions, and then use Install Local later. You could also write a script to load extensions. This is a waste of time, and new users will get confused.
There are also other boot methods which enable the usb drive to be accessed. Again, this makes it more complicated, and would confuse new users.