I follow the instruction posted in the OP's linked [article][1].  I consider the official official El Capitan dmg to be the `InstallMacOSX.dmg` file that can be downloaded from a link posted at the Apple website [How to get old versions of macOS][2]. I installed Windows 10 20H2 in a VirtualBox virtual machine, then installed [Transmac v14.4][3]. Basically, the OP's linked [article][1] instructions the user to select "Restore with Disk Image" from the `InstallMacOSX.dmg` file to a USB flash drive, as shown below.

[![enter image description here][4]][4]

I did this using a 16 GB virtual external USB drive. Next, I created an El Capitan virtual machine and tried booting from this virtual external USB drive. I found the drive was not bootable. Therefore, I believe the flash drive created by the OP would not appear in Mac Startup Manager on the OP's Mac when the <kbd>option</kbd> was held down at startup. The OP must have been selecting to boot from the internal drive which probably has been corrupted.

I would like to think the best and assume the [article][1] was not meant to be misleading. I assume referenced dmg file is not officially from Apple and probably would have worked. However, the [article][1] failed to include a link to this file.

  [1]: https://www.geekrar.com/create-bootable-usb-installer-for-mac-osx-el-capitan-on-windows/
  [2]: https://support.apple.com/en-us/HT211683
  [3]: https://www.acutesystems.com/scrtm.htm
  [4]: https://i.sstatic.net/VqvZz.png