I've seen hardware where you have to do multiple things: disable secure boot (which only allows booting from a trusted hard-disk or something along those line), enable something called CSM, enable USB at BIOS time (so you can boot from the USB stick or DVD/CD drive attached via USB), and disable "fast boot", which is something that actually prevents real booting and instead re-starts an already running windows copy.
In addition, I've seen a laptop (wasn't ASUS, an internal model built by my then-employer) where you can only boot from GPT formatted devices, not MBR ones. I have no idea how this plays with FreeBSD USB sticks, but we were able to get Linux to boot on such a machine.
Most likely, the person at Asus you talked to gave you incorrect information. It is difficult to imagine a machine that is so locked down that you can only (re-) install the OS that comes on the hard disk. It would run into all manner of logistics and legal problems.