Suspect it could boot from USB, just not using a typical bootable ISO.
I don't know the exact details but IIRC Pentium 4 era BIOS were really picky about USB capacity and logical layout. I bricked one USB stick trying to figure it all out, but the "good news" is that the BIOS on my PERL motherboard did detect the USB device, and the boot sector, and started loading stuff..
And it snagged on Windows OS boot loader step. The "bad news" is I kept trying and the end state was a USB disk that is now completely unreadable in all machines! 🤣
In summary, I was half way to being expert 😉
I think one of my big mistakes was trying to force the use of a new USB stick. Those early BIOS don't like big capacities. They also don't like multiple partitions. So, in preparation to try again I bought several old stock 2GB USB disks - but I haven't yet tried again.
Desktop timeline [ MOS 7501 → 68030 → x86(P5/MMX) → x86(K6-2) → x86(K7*) → PPC(G3*) → x86-64(K8) → x86-64(Xeon) → x86-64(i5) → x86-64(i7) ] * lost