Not all Mac are built the same. Your Mac is probably just a year to early. 

The industry switched to EFI booting of Windows in 2011. Apple did not catch up until 2012. This is also the year Apple starting to eliminate the optical drives from Mac models. However, since Windows 7 required a BIOS boot, Apple added the ability to BIOS boot from USB on models where the optical drive was eliminated. This continued though at least the 2014 model year. In 2015, Apple started to eliminate the ability to BIOS boot entirely, which effectively ended the ability to install Windows 7.

So your model probably can not BIOS boot from USB. However, the very next MacBook Pro model can BIOS boot from USB. For example, see the question [Bootcamp “Missing Operating System” on external drive][1].

One possible way to come close to having Windows on an USB drive would be to put the System partition on the internal drive and Windows partition on the USB drive. The System and Windows partitions are described in the Microsoft document [BIOS/MBR-based hard drive partitions][2]. I have never tested such an arrangement to see if it would work with any Mac model. You would have to try this on your Mac to see if it would work.


  [1]: https://apple.stackexchange.com/questions/377536/bootcamp-missing-operating-system-on-external-drive
  [2]: https://docs.microsoft.com/en-us/windows-hardware/manufacture/desktop/configure-biosmbr-based-hard-drive-partitions