For users to failing to flash the firmware, the USB port is not an option that will work. Is that correct?
If we talk about the Profilux 3 and not the 3.1N or 3.1A, the answer is "Yes, you need a PC with RS232 serial port".

An USB<>RS232 converter also may work, but it depends on this device if it does or not. Devices with FTDI chipset are working best, but there is no guarantee for that.